Migrate deprecated gitlab-runner to gitlab-ci-local

Local gitlab-runner exec was deprecated as of v16 (current version is v17).

This, despite a healthy pushback from the user community the last time they tried to remove it, and a new feature request with tons of input from premium Gitlab users hoping for the functionality to be restored.

Meanwhile, the 3rd party gitlab-ci-local Just Works, has a healthy build, good test and security badges, is currently maintained, etc. I'm using it to troubleshoot some CI issues in Rugged and Aegir, and it seems just fine so far.

I recommend we migrate to it from the legacy gitlab-runner which is now useless to us:

• Update Drumkit tests to expect the following changes...
• Add a make gitlab-ci-local target that downloads the latest release.
• Replace the make gitlab-runner target with a warning message about its deprecation status, links to gitlab-ci-local's docs and suggests calling make gitlab-ci-local instead.