Update HOWTO-RELEASE.md regarding auto_tag_stable.yml

Update the procedure to update the workflow that automatically refreshes the "stable" tag from commits in the current release branch.

HOWTO-RELEASE.md

@@ -149,7 +149,13 @@ Barring a successful promotion process we can proceed with the final release.
 
 See step 1.6 above. Do not rename the archives with RC postfixes.
 
-### 2.2 Update website
+
+### 2.2 Update the STABLE_BRANCH repository variable
+
+Go to https://github.com/OSGeo/PROJ/settings/variables/actions/STABLE_BRANCH
+and update the value to be x.y
+
+### 2.3 Update website
 
 - Add the release notes from `NEWS.md` to `docs/source/news.rst`
 
@@ -165,7 +171,7 @@ See step 1.6 above. Do not rename the archives with RC postfixes.
 
 When pushed upstream the website will be update to the current version.
 
-### 2.3 Tag the release
+### 2.4 Tag the release
 
 Tag the release with:
 
@@ -174,21 +180,21 @@ git tag -a -m "Create tag x.y.z" x.y.z
 git push --tags
 ```
 
-### 2.4 Check the GitHub Zeonodo integration
+### 2.5 Check the GitHub Zeonodo integration
 
 Go to https://zenodo.org/account/settings/github/ and check that the OSGeo/PROJ
 repository is ON so that the creation of the GitHub release triggers the
 creation of a corresponding Zenodo record.
 
-### 2.5 Prepare the release on GitHub
+### 2.6 Prepare the release on GitHub
 
 When the new tag is pushed upstream GitHub picks up that a new release has been
 issued. Update the new release on <https://github.com/OSGeo/PROJ/releases> with
 the release notes from `NEWS.md` and add the prepared source distribution archives
 to the release (otherwise GitHub will just package the entire repository - we
 don't want that).
 
-### 2.6 Announce the new release
+### 2.7 Announce the new release
 
 The release should be announced on PROJ and MetaCRS mailing lists. Additionally
 the release announcement should be sent to <[email protected]> which will add