patch(_sync_docs.yaml): Update source of gatekeeper action (#201)

## Issue
The `_sync_docs.yaml` workflow was using a fork instead of the official
canonical repository for the discourse-gatekeeper action.

The canonical action no longer has the limitations that prompted using
the fork.

## Solution
Changed the repository of the action to
`canonical/discourse-gatekeeper@main`

**Note:** Using `@main` instead of their recommended `@stable` because
those branches have diverged for unknown reasons, and `@main` seems to
have some fixes that we need.

.github/workflows/_sync_docs.yaml

@@ -17,7 +17,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
       - name: Open PR with changes to Discourse topics
-        uses: deusebio/discourse-gatekeeper@c8adb89ea1cbceca54d78da798658373615487ac
+        uses: canonical/discourse-gatekeeper@main
         id: docs-pr
         with:
           discourse_host: discourse.charmhub.io