Add auto-merge GitHub workflow (#249)

We currently have to maintain Dependabot updates across 5 repos for the [Charging Module team](https://github.com/DEFRA/sroc-service-team).

All we do when checking a Dependabot update is confirm it hasn't broken CI. We then approve and merge.

We are not really adding any value to the process. So, to reduce the maintenance load on the team we have agreed that any Dependabot update that passes our CI build can be automatically merged.

This adds [Dependabot Auto Merge](https://github.com/marketplace/actions/dependabot-auto-merge) to the repo which is a GitHub action that should handle this for us.

.github/workflows/auto-merge.yml

@@ -0,0 +1,14 @@
+name: auto-merge
+
+on:
+  pull_request:
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: ahmadnassri/action-dependabot-auto-merge@v2
+        with:
+          target: minor
+          github-token: ${{ secrets.AUTO_MERGE }}