Remove support for array of strings when defining services for Docker Executor (!2035) · Merge requests · GitLab.org / gitlab-runner · GitLab

What does this MR do?

Removes deprecated support for array of strings when defining services for Docker Executor.

Why was this MR needed?

See #4922 (closed)

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

Does this MR meet the acceptance criteria?

Documentation created/updated
Added tests for this feature/bug
In case of conflicts with master - branch was rebased

What are the relevant issue numbers?

Closes #4922 (closed)