ci(workflows): update dependabot auto-merge workflow

The dependabot auto-merge workflow has been updated to use a new action and simplified logic for merging pull requests.

- **Enhancements & Refinements**:
  - Replaced `dependabot/fetch-metadata` and `peter-evans/enable-pull-request-automerge` with `fastify/github-action-merge-dependabot`
  - Simplified the workflow by removing conditional logic for update types
  - Updated the condition syntax to use explicit expression syntax
  - Added explicit `github-token` configuration for the new action
```

.github/workflows/dependabot-auto-merge.yml

@@ -10,18 +10,11 @@ permissions:
 
 jobs:
   dependabot:
-    if: github.actor == 'dependabot[bot]'
+    if: ${{github.actor == 'dependabot[bot]' }}
     runs-on: ubuntu-latest
 
     steps:
-      - name: Fetch Dependabot metadata
+      - name: Merge PR from dependabot
-        id: metadata
+        uses: fastify/github-action-merge-dependabot@v3
-        uses: dependabot/fetch-metadata@v2
-
-      - name: Enable auto-merge (squash)
-        if: |
-          steps.metadata.outputs.update-type == 'version-update:semver-minor' ||
-          steps.metadata.outputs.update-type == 'version-update:semver-patch'
-        uses: peter-evans/enable-pull-request-automerge@v3
         with:
-          merge-method: squash
+          github-token: '${{ secrets.GITHUB_TOKEN }}'