Explain why migrate_user_mentions FF is not removed yet

config/feature_flags/development/migrate_user_mentions.yml

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+# While this FF is enabled by default, we want to keep it in place for now as there are still related migrations that
+# have not been run in production yet. For additional info, see https://gitlab.com/gitlab-org/gitlab/-/issues/21801
+# Discussion and removal MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60322
 name: migrate_user_mentions
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34378
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/231175