Add a note to release docs to ensure green CI (Leaflet#8668)

Co-authored-by: Florian Bischof <[email protected]>

RELEASE.md

@@ -2,6 +2,7 @@
 
 - [ ] Ensure all https://github.com/Leaflet/Leaflet/labels/blocker issues and pull requests are resolved.
 - [ ] Update [the changelog](https://github.com/Leaflet/Leaflet/blob/main/CHANGELOG.md) since last release and commit.
+- [ ] Make sure all tests pass on CI before proceeding.
 - [ ] Run `npm version <patch | minor | major>` (this will bump the version in `package.json` and create a new tag).
 - [ ] Run `git push --follow-tags` to push the commit created by NPM to Github (together with the tag).
 - [ ] Wait for the CI to complete and follow the logs to make sure it runs successfully.