Update rubocop-on-rbs and use plugins

[ksss]

Both rubocop-rubycw and rubocop-on-rbs plugins are supported.
Using plug-ins reduces warnings when using rubocop.

rubocop-rubycw extension supports plugin, specify `plugins: rubocop-rubycw` instead of `require: rubocop-rubycw` in /home/runner/work/rbs/rbs/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.
rubocop-on-rbs extension supports plugin, specify `plugins: rubocop-on-rbs` instead of `require: rubocop-on-rbs` in /home/runner/work/rbs/rbs/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.

See also: https://docs.rubocop.org/rubocop/plugin_migration_guide.html