Simplify the codequality job

What does this MR do?

This improves the codequality job to not rely on a custom codeclimate Docker image but instead, tag a codeclimate-rubocop image from our custom codeclimate-rubocop which includes gitlab-styles.

Are there points in the code the reviewer needs to double check?

I've added a GitlabSecurity/PublicSend offense to ensure that custom cops from gitlab-styles (included in our custom codeclimate-rubocop) are effectively used. I will obviously remove this commit before merging.

Why was this MR needed?

To simplify our CI setup to not have to build / tag / maintain a custom codeclimate image.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?