Remove support for Rails v5.2

Rails v5.2 is EOL since 01 Jun 2022 [1].

Our new release policy [2] makes it clear that we don't support Rails
versions that are no longer maintained.

[1] - https://endoflife.date/rails
[2] - https://solidus.io/release_policy/

.circleci/config.yml

@@ -343,9 +343,5 @@ workflows:
           context: slack-secrets
           name: *name
           matrix: { parameters: { rails: ['6.0'], ruby: ['2.7'], database: ['sqlite'], paperclip: [true] } }
-      - test_solidus:
-          context: slack-secrets
-          name: *name
-          matrix: { parameters: { rails: ['5.2'], ruby: ['2.7'], database: ['sqlite'], paperclip: [true] } }
       - dev_tools:
           context: slack-secrets

core/lib/spree/core.rb

@@ -84,15 +84,6 @@ class GatewayError < RuntimeError; end
   end
 end
 
-if Gem::Version.new(Rails.version) < Gem::Version.new('6.0')
-  Spree::Deprecation.warn <<~HEREDOC
-    Rails 5.2 (EOL) is deprecated and will not be supported anymore from the next Solidus version.
-    Please, upgrade to a more recent Rails version.
-    Read more on upgrading from Rails 5.2 to Rails 6.0 here:
-    https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-5-2-to-rails-6-0
-  HEREDOC
-end
-
 require 'spree/core/version'
 
 require 'spree/core/active_merchant_dependencies'

core/solidus_core.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |s|
     actionmailer actionpack actionview activejob activemodel activerecord
     activesupport railties
   ].each do |rails_dep|
-    s.add_dependency rails_dep, ['>= 5.2', '< 7.1.x']
+    s.add_dependency rails_dep, ['>= 6.0', '< 7.1.x']
   end
 
   s.add_dependency 'activemerchant', '~> 1.66'