Add details on release branch to issue template

.gitlab/issue_templates/release.md

 # Release checklist:
 
+* [ ] If this is a major release, create a new release branch (`v0.4`, etc.).
 * [ ] update/review `docs/*` and make sure it matches the current state
   * Update how to checkout the latest stable tag
-* [ ] Ensure all dependencies are updated and locked
+* [ ] Ensure all dependencies are updated and locked (locking should be done
+    only on the release branch):
   * [ ] `requirements-stable.lock`
-  * [ ] `flux/*`
+  * [ ] chart versions in `flux/*`
+  * [ ] in `ansible/group_vars/all/oas.yml`: `flux_source.branch` should be set to the release branch
 * [ ] update [CHANGELOG.md](https://keepachangelog.com/en/1.0.0/)
   * Include `Known issues`
 * [ ] update the version number in the `VERSION` file