Adds a note to not re-assign to the bot until MRs are ready

.gitlab/merge_request_templates/Security Release.md

@@ -27,7 +27,7 @@ See [the general developer security release guidelines](https://gitlab.com/gitla
 
 ## Maintainer checklist
 - [ ] Correct milestone is applied and the title is matching across all backports
-- [ ] Assigned to `@gitlab-release-tools-bot` with passing CI pipelines
+- [ ] Assigned to `@gitlab-release-tools-bot` with passing CI pipelines and **when all backports including the MR targeting master are ready.**
 
 /label ~security