Add version text for new feature

doc/raketasks/cleanup.md

-# Cleanup
+# Clean up
+
+GitLab provides Rake tasks for cleaning up GitLab instances.
 
 ## Remove unreferenced LFS files from filesystem
 
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/36628) in GitLab 12.10.
+
 DANGER: **Danger:**
 Do not run this within 12 hours of a GitLab upgrade. This is to ensure that all background migrations have finished, which otherwise may lead to data loss.