activerecord: Rename AR::Relation::ClassMethods to AR::Base::ClassMethods

[tk0miya]

In #413, we added ActiveRecord::Relation::ClassMethods as a new styled component of _ActiveRecord_Relation_ClassMethods. But this name is not good because it is used to enhance the subclasses of ActiveRecord::Base, not the subclasses of ActiveRecord::Relation.

It should be renamed to ActiveRecord::Base::ClassMethods.

refs:

• https://github.com/pocke/rbs_rails/blob/v0.12.0/lib/rbs_rails/active_record.rb#L33
• https://github.com/ruby/gem_rbs_collection/pull/413