Change load_async return type to self (#997)

Update `load_async` to return `self` instead of the explicit
`ActiveRecord::Relation[Model]`.

While both represent the same runtime object, RBS treats them as
distinct types from `Model::ActiveRecord_Relation` (returned by
methods like `where` or `all`). This discrepancy causes type
mismatches and forces unnecessary type distinctions during method
chaining or when defining signatures. Changing it to `self` ensures
consistency with other relation methods.

Author: rhiroe

gems/activerecord/7.2/activerecord-7.2.rbs

@@ -32,7 +32,7 @@ module ActiveRecord
     def insert_all!: (untyped attributes, ?returning: untyped?, ?record_timestamps: bool?) -> untyped
     def upsert: (untyped attributes, ?unique_by: untyped?, ?returning: untyped?, ?record_timestamps: bool?) -> untyped
     def upsert_all: (untyped attributes, ?unique_by: untyped?, ?returning: untyped?, ?record_timestamps: bool?) -> untyped
-    def load_async: () -> Relation
+    def load_async: () -> self
   end
 
   module Inheritance

gems/activerecord/8.0/activerecord-8.0.rbs

@@ -32,7 +32,7 @@ module ActiveRecord
     def insert_all!: (untyped attributes, ?returning: untyped?, ?record_timestamps: bool?) -> untyped
     def upsert: (untyped attributes, ?unique_by: untyped?, ?returning: untyped?, ?record_timestamps: bool?) -> untyped
     def upsert_all: (untyped attributes, ?unique_by: untyped?, ?returning: untyped?, ?record_timestamps: bool?) -> untyped
-    def load_async: () -> Relation
+    def load_async: () -> self
   end
 
   module Inheritance