Add warning to stop Puma and Sidekiq when restoring from backup

As we saw in https://gitlab.com/gitlab-org/gitlab/-/issues/36405, having
processes that can write to GitLab tables while restoring from a
database backup can cause duplicate rows be inserted and schema changes
to fail quietly. At the moment, we don't have a good story of restoring
a database backup in a single transaction, so we need to tell admins to
stop Puma and Sidekiq.

changelogs/unreleased/sh-document-stop-sidekiq-puma-backup-restore.yml

+---
+title: Add warning to stop Puma and Sidekiq when restoring from backup
+merge_request: 40791
+author:
+type: other

lib/tasks/gitlab/backup.rake

@@ -47,6 +47,11 @@ namespace :gitlab do
         begin
           unless ENV['force'] == 'yes'
             warning = <<-MSG.strip_heredoc
+              Be sure to stop Puma, Sidekiq, and any other process that
+              connects to the database before proceeding. For Omnibus
+              installs, see the following link for more information:
+              https://docs.gitlab.com/ee/raketasks/backup_restore.html#restore-for-omnibus-gitlab-installations
+
               Before restoring the database, we will remove all existing
               tables to avoid future upgrade problems. Be aware that if you have
               custom tables in the GitLab database these tables and all data will be