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

…hods

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
* #413

gems/activerecord/6.0/activerecord.rbs

@@ -367,7 +367,9 @@ module ActiveRecord
                 | () { (Model) -> boolish } -> Array[Model]
       def reselect: (*Symbol | String) -> self
     end
+  end
 
+  class Base
     module ClassMethods[Model, Relation, PrimaryKey]
       def all: () -> Relation
       def ids: () -> Array[PrimaryKey]