ci(dependabot): merge directly via squash instead of use-github-auto-merge

The previous config required branch protection with required status checks,
but main has no protection rules configured, so auto-merge never fired and
PRs had to be merged manually.

Switch to direct squash merge in the action itself, which works regardless
of branch protection settings.

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

@@ -18,5 +18,5 @@ jobs:
         uses: fastify/github-action-merge-dependabot@v3
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
-          use-github-auto-merge: true
           target: minor
+          merge-method: squash