Prefer `connection` over `ApplicationRecord.connection` in helpers

Use the existing `connection` helper in migrations, instead of
`ApplicationRecord.connection`.

See
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/82565#note_870365811.

lib/gitlab/database/async_indexes/migration_helpers.rb

@@ -72,7 +72,7 @@ module Gitlab
         end
 
         def async_index_creation_available?
-          ApplicationRecord.connection.table_exists?(:postgres_async_indexes)
+          connection.table_exists?(:postgres_async_indexes)
         end
       end
     end