Add `deploy-cleanup-old-deleted-files` scheduled job

What does this MR do?

• Add deploy-cleanup-old-deleted-files scheduled job (which can also be run manually on-demand)
  • Supports independent per-site production deploys by removing need for --delete flag on master GCP deploy rsync.
  • Introduces a separate "deploy-cleanup-old-deleted-files" scheduled job which does a full build of everything and deploys with the DEPLOY_CLEANUP_OLD_DELETED_FILES flag set.
  • This will use all artifacts from partial build jobs to ensure everything is still cleaned up, even if partial build jobs are added/deleted/refactored, and when top-level paths of the deploy are deleted/renamed.
  • As part of this, the staging app deploy will be changed to a manual job. See discussion on slack. Also update related docs.
  • This also resolves #7661 (closed)

Tasks

• Write job
• Remove DEPLOY_CLEANUP_OLD_DELETED_FILES flag from deploy
  • ...and add to deploy_staging job and make it invoked manually (see slack thread)
• Update docs about staging app deploy being switched to manual: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/doc/redirects_on_about_gitlab_com.md
• Set up schedule in CI config (Daily at 2PM UTC to start, can be changed to more frequently if needed).
• Ensure it can be triggered manually too
• Test with fake config in MR
• Review cleanup job in prod after merge (Successful run via scheduled job: https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/651326382)
• Test manual staging deploy in prod after merge (https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/651326376)

Related issues

• Relates #8302 (closed)
• Resolves #7661 (closed)
• Resolves #8337 (closed)