Migrate to frequenz-floss dependabot-auto-approve action

Use commit hash instead of version tag for better security and
reproducibility.

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

.github/workflows/auto-dependabot.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.actor == 'dependabot[bot]'
     steps:
-      - uses: ad/dependabot-auto-approve@v1
+      - uses: frequenz-floss/dependabot-auto-approve@3cad5f42e79296505473325ac6636be897c8b8a1 # v1.3.2
         with:
           dependency-type: 'all'
           auto-merge: 'true'