feat: make server shutdown timeout configurable

The following discussion from !664 (merged) should be addressed:

• @jaime started a discussion:

  // TODO: make this timeout configurable

Proposal

Add a new config flag server-shutdown-timeout of type time.Duration.

• Add to Pages server
• Add to Omnibus config (example omnibus-gitlab!5832 (diffs))
• Add to GitLab Charts (example gitlab-org/charts/gitlab!2338 (merged))
• Document in the admin docs (example gitlab!77969 (merged))