Update to govuk-dependabot-merger v2 and automerge external deps

This repo has:

- Sufficient test coverage (~99%)
- [Security scans in place](https://github.com/alphagov/govuk_app_config/blob/main/.github/workflows/ci.yml#L14-L29)
- No manual deployment step
- Branch protection rules in place, ensuring the tests and security
  scans pass (as of https://trello.com/c/3CYpdFw4/3480-set-stricter-branch-protection-defaults)

.govuk_dependabot_merger.yml

@@ -1,10 +1,4 @@
-api_version: 1
-auto_merge:
-  - dependency: rubocop-govuk
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: plek
-    allowed_semver_bumps:
-      - patch
-      - minor
+api_version: 2
+defaults:
+  auto_merge: true
+  update_external_dependencies: true