Use RuboCop Plugin

Follow-up to rails/rails#54678.

This PR suppresses the folloiwng RuboCop's warnings:

```console
$ bundle exec rubocop
rubocop-performance extension supports plugin, specify `plugins: rubocop-performance` instead of
`require: rubocop-performance` in /Users/koic/src/github.com/rsim/oracle-enhanced/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.

rubocop-rails extension supports plugin, specify `plugins: rubocop-rails` instead of
`require: rubocop-rails` in /Users/koic/src/github.com/rsim/oracle-enhanced/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.

rubocop-rspec extension supports plugin, specify `plugins: rubocop-rspec` instead of
`require: rubocop-rspec` in /Users/koic/src/github.com/rsim/oracle-enhanced/.rubocop.yml.
For more information, see https://docs.rubocop.org/rubocop/plugin_migration_guide.html.
```

Please see here for details:
https://docs.rubocop.org/rubocop/plugin_migration_guide.html

Author: koic

.rubocop.yml

@@ -1,4 +1,4 @@
-require:
+plugins:
   - rubocop-performance
   - rubocop-rails
   - rubocop-rspec