Refine ActiveRecord::Base instead of Object

Don't refine classes that it isn't necessary.
As a technical issue, overriding Object#dup causes a problem of stopping
`ActiveRecord::Base.establish_connection` in Ruby 3.0.
I will try to continue to investigate this issue.

lib/activerecord-bitemporal/bitemporal.rb

@@ -327,7 +327,7 @@ def update_transaction_to(value)
           end
         end
 
-        refine Object do
+        refine ActiveRecord::Base do
           # MEMO: Do not copy `swapped_id`
           def dup(*)
             super.tap { |itself|