Extend Rails/FindBy Rubocop to more folders

What does this MR do?

Extend Rails/FindBy cop to more folders.

We only covers /ee ones is this project config while /app and /lib will be inherited from gitlab-styles gem.

The final Rails/FindBy cop config is as followed

bundle exec rubocop --show-cops Rails/FindBy

# Supports --auto-correct
Rails/FindBy:
  Description: Prefer find_by over where.first.
  StyleGuide: https://rails.rubystyle.guide#find_by
  Enabled: true
  VersionAdded: '0.30'
  Include:
  - app/models/**/*.rb
  - app/**/*.rb
  - lib/**/*.rb
  - ee/app/**/*.rb
  - ee/lib/**/*.rb
  - spec/**/*.rb
  - ee/spec/**/*.rb

Related issue #212410 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team