Automatically remove outdated generated sources preview branches

After PR is merged or closed, we can trigger a task that will remove all matching source preview branches.
To detect matching branches, `build_preview_generated_code` should include some identifier related to PR in their names.
Something like `branch_name=generated-sources/docs-update-${{ github.event.number }}-${{ github.run_number }}`
https://stackoverflow.com/questions/59077079/how-to-get-pull-request-number-within-github-actions-workflow

and removing them:
```
branch_prefix="generated-sources/docs-update-${{ github.event.number }}"
for branch in $(git branch -r | grep "$branch_prefix"); do
branch_name=${branch#origin/}
git push origin --delete "$branch_name"
done
```

Start by locating the GitHub Actions task named build_preview_generated_code and the workflow triggered when a pull request is merged or closed. Read how preview branch names are currently generated, then verify the cleanup task can identify matching branches and delete them safely. Done means outdated generated-sources preview branches are removed after the related pull request ends.

Written by the indexing model from the issue text.