Gatekeeper stable test (#260)

Reverting the gatekeeper action to the recommended `stable` branch in
order to attempt re-importing all docs and ruling out an issue coming
from the `main` branch.

Author: a-velasco

.github/workflows/sync_docs.yaml

@@ -18,7 +18,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Open PR with docs changes
-        uses: canonical/discourse-gatekeeper@main
+        uses: canonical/discourse-gatekeeper@stable
         id: docs-pr
         with:
           discourse_host: discourse.charmhub.io