Add ci_runners_stale_group_runners_prune_worker_cron setting

What does this MR do?

This MR adds a EE-only cronjob for a worker that will regularly prune stale CI runners from the database.

Related issues

gitlab#361112 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on the dev.gitlab.org mirror for this branch if the change is touching anything besides documentation or internal cookbooks. Please note that feature branches are not automatically mirrored to dev.gitlab.org, and should be manually pushed whenever necessary.
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
  • I've looked at past commits adding settings to this area and they don't seem to have documentation updates
• Tests added
  • I've looked at past commits adding settings to this area and they don't seem to add new tests
• Integration tests added to GitLab QA
  • I don't know whether this is something we test in GitLab QA
• Equivalent MR/issue for the GitLab Chart opened: gitlab-org/charts/gitlab!2565 (merged)
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.
  • I don't think anything special needs to be done here