Make Rails/CreateTableWithTimestamps respect active_storage_variant_records table

[koic]

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.

% 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.

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
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