Remove ForeignKeys module which was introduced at #32299

To solve the problem #32299, just enough to introduce
`fk_ignore_pattern` option.

I don't think there is a need to expose these constants.

activerecord/lib/active_record.rb

@@ -43,7 +43,6 @@ module ActiveRecord
   autoload :DatabaseConfigurations
   autoload :DynamicMatchers
   autoload :Enum
-  autoload :ForeignKeys
   autoload :InternalMetadata
   autoload :Explain
   autoload :Inheritance

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -1324,7 +1324,7 @@ module ActiveRecord
             identifier = "#{table_name}_#{options.fetch(:column)}_fk"
             hashed_identifier = Digest::SHA256.hexdigest(identifier).first(10)
 
-            "#{ActiveRecord::ForeignKeys::PREFIX}_#{hashed_identifier}"
+            "fk_rails_#{hashed_identifier}"
           end
         end
 

activerecord/lib/active_record/foreign_keys.rb

@@ -1,12 +0,0 @@
-# frozen_string_literal: true
-
-module ActiveRecord
-  module ForeignKeys
-    # The prefix used by Rails to name unnamed foreign keys.
-    PREFIX = "fk_rails"
-
-    # Default regular expression used by Rails to determine if a foreign key
-    # name was generated.
-    DEFAULT_IGNORE_PATTERN = /^#{PREFIX}_[0-9a-f]{10}$/
-  end
-end

activerecord/lib/active_record/schema_dumper.rb

@@ -21,7 +21,7 @@ module ActiveRecord
     # :singleton-method:
     # Specify a custom regular expression matching foreign keys which name
     # should not be dumped to db/schema.rb.
-    cattr_accessor :fk_ignore_pattern, default: ActiveRecord::ForeignKeys::DEFAULT_IGNORE_PATTERN
+    cattr_accessor :fk_ignore_pattern, default: /^fk_rails_[0-9a-f]{10}$/
 
     class << self
       def dump(connection = ActiveRecord::Base.connection, stream = STDOUT, config = ActiveRecord::Base)