Document how to remove to_s deprecation warnings when defaul format is changed

Closes #48995.

When users change `Time::DATE_FORMATS[:default]` or
`Date::DATE_FORMATS[:default]` we are now showing a deprecation message
when calling `to_s`. Users can fix this deprecation message in their own
code. But there are a few other implicit callers of `to_s` that
they can't change like JSON serialization.

The only way for users to disable this deprecation message, at this
point is to mark their application as safe to use the default Ruby
behavior for `to_s` with an environment variable.

We can't use the config because some gems will likely require the
core extensions before the config is loaded.

Author: rafaelfranca

actioncable/lib/action_cable/connection/base.rb

@@ -237,15 +237,15 @@ def started_request_message
             request.filtered_path,
             websocket.possible? ? " [WebSocket]" : "[non-WebSocket]",
             request.ip,
-            Time.now.to_s ]
+            Time.now.to_default_s ]
         end
 
         def finished_request_message
           'Finished "%s"%s for %s at %s' % [
             request.filtered_path,
             websocket.possible? ? " [WebSocket]" : "[non-WebSocket]",
             request.ip,
-            Time.now.to_s ]
+            Time.now.to_default_s ]
         end
 
         def invalid_request_message

activejob/lib/active_job/log_subscriber.rb

@@ -38,7 +38,7 @@ def enqueue_at(event)
         end
       else
         info do
-          "Enqueued #{job.class.name} (Job ID: #{job.job_id}) to #{queue_name(event)} at #{scheduled_at(event)}" + args_info(job)
+          "Enqueued #{job.class.name} (Job ID: #{job.job_id}) to #{queue_name(event)} at #{scheduled_at(event).to_default_s}" + args_info(job)
         end
       end
     end

activesupport/lib/active_support/core_ext/date/deprecated_conversions.rb

@@ -16,9 +16,12 @@ def to_s(format = NOT_SET) # :nodoc:
       end
     elsif format == NOT_SET
       if formatter = DATE_FORMATS[:default]
-        ActiveSupport::Deprecation.warn(
-          "Using a :default format for Date#to_s is deprecated. Please use Date#to_fs instead."
-        )
+        ActiveSupport::Deprecation.warn(<<-MSG.squish)
+          Using a :default format for Date#to_s is deprecated. Please use Date#to_fs instead. If you fixed all places
+          inside your application that you see this deprecation, you can set
+          `ENV['RAILS_DISABLE_DEPRECATED_TO_S_CONVERSION']` to `"true"` in the `config/application.rb` file before
+          the `Bundler.require` call to fix all the callers outside of your application.
+        MSG
         if formatter.respond_to?(:call)
           formatter.call(self).to_s
         else

activesupport/lib/active_support/core_ext/date_time/deprecated_conversions.rb

@@ -12,9 +12,12 @@ def to_s(format = NOT_SET) # :nodoc:
       formatter.respond_to?(:call) ? formatter.call(self).to_s : strftime(formatter)
     elsif format == NOT_SET
       if formatter = ::Time::DATE_FORMATS[:default]
-        ActiveSupport::Deprecation.warn(
-          "Using a :default format for DateTime#to_s is deprecated. Please use DateTime#to_fs instead."
-        )
+        ActiveSupport::Deprecation.warn(<<-MSG.squish)
+          Using a :default format for DateTime#to_s is deprecated. Please use DateTime#to_fs instead. If you fixed all
+          places inside your application that you see this deprecation, you can set
+          `ENV['RAILS_DISABLE_DEPRECATED_TO_S_CONVERSION']` to `"true"` in the `config/application.rb` file before
+          the `Bundler.require` call to fix all the callers outside of your application.
+        MSG
         if formatter.respond_to?(:call)
           formatter.call(self).to_s
         else

activesupport/lib/active_support/core_ext/range/deprecated_conversions.rb

@@ -11,9 +11,12 @@ def to_s(format = NOT_SET)
         formatter.call(first, last)
       elsif format == NOT_SET
         if formatter = RangeWithFormat::RANGE_FORMATS[:default]
-          ActiveSupport::Deprecation.warn(
-            "Using a :default format for Range#to_s is deprecated. Please use Range#to_fs instead."
-          )
+          ActiveSupport::Deprecation.warn(<<-MSG.squish)
+            Using a :default format for Range#to_s is deprecated. Please use Range#to_fs instead. If you fixed all
+            places inside your application that you see this deprecation, you can set
+            `ENV['RAILS_DISABLE_DEPRECATED_TO_S_CONVERSION']` to `"true"` in the `config/application.rb` file before
+            the `Bundler.require` call to fix all the callers outside of your application.
+          MSG
           formatter.call(first, last)
         else
           super()

activesupport/lib/active_support/core_ext/time/deprecated_conversions.rb

@@ -12,9 +12,12 @@ def to_s(format = NOT_SET) # :nodoc:
       formatter.respond_to?(:call) ? formatter.call(self).to_s : strftime(formatter)
     elsif format == NOT_SET
       if formatter = ::Time::DATE_FORMATS[:default]
-        ActiveSupport::Deprecation.warn(
-          "Using a :default format for Time#to_s is deprecated. Please use Time#to_fs instead."
-        )
+        ActiveSupport::Deprecation.warn(<<-MSG.squish)
+          Using a :default format for Time#to_s is deprecated. Please use Time#to_fs instead. If you fixed all places
+          inside your application that you see this deprecation, you can set
+          `ENV['RAILS_DISABLE_DEPRECATED_TO_S_CONVERSION']` to `"true"` in the `config/application.rb` file before
+           the `Bundler.require` call to fix all the callers outside of your application.
+        MSG
         if formatter.respond_to?(:call)
           formatter.call(self).to_s
         else

activesupport/lib/active_support/time_with_zone.rb

@@ -222,9 +222,12 @@ def to_s(format = NOT_SET)
         formatter.respond_to?(:call) ? formatter.call(self).to_s : strftime(formatter)
       elsif format == NOT_SET
         if formatter = ::Time::DATE_FORMATS[:default]
-          ActiveSupport::Deprecation.warn(
-            "Using a :default format for TimeWithZone#to_s is deprecated. Please use TimeWithZone#to_fs instead."
-          )
+          ActiveSupport::Deprecation.warn(<<-MSG.squish)
+            Using a :default format for TimeWithZone#to_s is deprecated. Please use TimeWithZone#to_fs instead.
+            If you fixed all places inside your application that you see this deprecation, you can set
+            `ENV['RAILS_DISABLE_DEPRECATED_TO_S_CONVERSION']` to `"true"` in the `config/application.rb` file before
+            the `Bundler.require` call to fix all the callers outside of your application.
+          MSG
           formatter.respond_to?(:call) ? formatter.call(self).to_s : strftime(formatter)
         else
           "#{time.strftime("%Y-%m-%d %H:%M:%S")} #{formatted_offset(false, 'UTC')}" # mimicking Ruby Time#to_s format