Fixes Rubocop require/plugin warning

When running rubocop, this warning is printed:

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

To fix it, we need to add that library as a plugin: https://docs.rubocop.org/rubocop/latest/plugin_migration_guide.html#for-plugin-users

Author: stefannibrasil

.rubocop.yml

@@ -1,9 +1,9 @@
 require:
   - standard
-  - rubocop-factory_bot
 
 plugins:
   - rubocop-capybara
+  - rubocop-factory_bot
   - rubocop-rspec
   - rubocop-rspec_rails