Update auto-merge workflow to use hashes (#96)

Use commit hash for `frequenz-floss/dependabot-auto-approve` action for
better security and reproducibility.

Author: Marenz

.github/workflows/auto-dependabot.yaml

@@ -9,7 +9,7 @@ jobs:
     if: github.actor == 'dependabot[bot]'
     steps:
       - name: Auto-approve and merge Dependabot PRs
-        uses: frequenz-floss/[email protected]
+        uses: frequenz-floss/dependabot-auto-approve@005e52004f5d5c6af2f81b89ec25e5cf6f3dfd77 # v1.3.0
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           merge-method: merge