Remove project setting for CI coverage detection

Ref: #17633 (closed)

What does this MR do and why?

This MR removes support for coverage CI detection at the project level.

This is a breaking change and users are required to use the coverage keyword in .gitlab-ci.yml to continue using this feature.

This MR does the following:

• Ignores project#build_coverage_regex with ignored_columns
• Prevents setting coverage_regex at the project setting level
• Drops associated QA & specs no longer needed

Why are we doing this?

To simplify our product it was decided to only support this feature with a job in our .gitlab-ci.yml configuration file.

Testing locally

1. Try to update the project#build_coverage_regex of a given project
2. Verify that it's impossible to update the data at the project level
3. Verify the form is not being displayed in the UI 👉 http://127.0.0.1:3000/root/<PROJECT_ID>/-/settings/ci_cd

Project.last.update!(build_coverage_regex: 'something')
=> ActiveModel::UnknownAttributeError: unknown attribute 'build_coverage_regex' for Project.

Screenshots

Here a screenshot of how the feature used to look like below 👇

Impact of this change

• Are there existing users depending on this feature?
  • On gitlab.com we have identified ~350K projects being affected
  • Impossible to say for self-managed users.
• Was sufficient time given to communicate the change?
  • This was announced in %14.8

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.