Add ci_runners_stale_machines_cleanup_worker cronjob

What does this MR do?

This MR follows up on gitlab-org/gitlab!108837 (merged) to add the ci_runners_stale_machines_cleanup_worker setting to the Omnibus package.

Related issues

gitlab-org/gitlab#387398 (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
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened (Add ci_runners_stale_machines_cleanup_worker se... (gitlab-org/omnibus-gitlab!6623 - 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.