Remove project setting for ci coverage detection

Release notes

To simplify the specification of a coverage pattern, in 15.0 the project setting for Test Coverage is being removed. Using the project’s .gitlab-ci.yml, provide a regular expression using the coverage keyword to set Testing Coverage in Merge Requests.

Background:

Since we can specify the coverage pattern in gitlab-ci.yml, we don't need the project setting any more.

What is the backstory of this project and how does it impact the approach?

Currently we have two places to set up the coverage pattern, which is a duplicate setting:

• project settings -> CI/CD Pipelines -> Test coverage parsing
• in gitlab-ci.yml

What do you already know about the areas you are exploring?

@godfat commented in gitlab-ce#27911:

I think it makes sense to remove that feature, but I am worried that it would break people's workflow. If we're removing it, we'll need to do it in major release and notify people beforehand.

What does success look like at the end of the project?

• no duplicate settings
• changes to gitlab-ci.yml are synced to forks of the project - project settings not

Links / references:

gitlab-ce#27911, gitlab-ce#28748