Make Rails/CreateTableWithTimestamps respect active_storage_variant_records table

This PR makes `Rails/CreateTableWithTimestamps` respect `active_storage_variant_records` table of
`db/migrate/*_create_active_storage_tables.active_storage.rb` auto-generated by `bin/rails active_storage:install`
even if `created_at` is not specified.

It suppresses the following offense.

```console
% bin/rails -v
Rails 6.1.4.1

% bin/rails active_storage:install
Copied migration
20210929095151_create_active_storage_tables.active_storage.rb from active_storage

% bundle exec rubocop --only Rails/CreateTableWithTimestamps
Inspecting 1 file
C

Offenses:

db/migrate/20210929095151_create_active_storage_tables.active_storage.rb:28:5:
C: Rails/CreateTableWithTimestamps: Add timestamps when creating a new table.
create_table :active_storage_variant_records do |t| ...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected
```

As far as I've heard from Rails committers, `active_storage_variant_records` table hasn't timestamp columns,
probably as a result of discussions in the Rails community. If you want to consider the need,
please discuss it with https://discuss.rubyonrails.org and give us feedback.

Author: koic

changelog/change_make_create_table_with_timestamps_aware_of_ast_variant_records.md

@@ -0,0 +1 @@
+* [#570](https://github.com/rubocop/rubocop-rails/pull/570): Make `Rails/CreateTableWithTimestamps` respect `active_storage_variant_records` table of `db/migrate/*_create_active_storage_tables.active_storage.rb` auto-generated by `bin/rails active_storage:install` even if `created_at` is not specified. ([@koic][])

config/default.yml

@@ -201,6 +201,10 @@ Rails/CreateTableWithTimestamps:
   VersionAdded: '0.52'
   Include:
     - db/migrate/*.rb
+  Exclude:
+    # Respect the `active_storage_variant_records` table of `*_create_active_storage_tables.active_storage.rb`
+    # auto-generated by `bin/rails active_storage:install` even if `created_at` is not specified.
+    - db/migrate/*_create_active_storage_tables.active_storage.rb
 
 Rails/Date:
   Description: >-

docs/modules/ROOT/pages/cops_rails.adoc

@@ -923,6 +923,10 @@ end
 | Include
 | `db/migrate/*.rb`
 | Array
+
+| Exclude
+| `db/migrate/*_create_active_storage_tables.active_storage.rb`
+| Array
 |===
 
 == Rails/Date