Update .govuk_dependabot_merger.yml to v2

As per RFC 167's [Conditions required for automatic patching](https://github.com/alphagov/govuk-rfcs/blob/main/rfc-167-auto-patch-dependencies.md#conditions-required-for-automatic-patching), this repo has:

- Sufficient test coverage ✅
- Sufficient security scanning ✅ 
- No manual deployment step ✅ 
- Branch protection rules in place ✅ 
- Been configured to only merge patch/minor updates ✅ 

Trello: https://trello.com/c/0FUL9Cv7/2541-upgrade-our-publishing-apps-to-govuk-dependabot-merger-v2

.govuk_dependabot_merger.yml

@@ -1,38 +1,7 @@
-api_version: 1
-auto_merge:
-  - dependency: gds-api-adapters
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: gds-sso
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govspeak
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_admin_template
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_app_config
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_publishing_components
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_test
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: plek
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: rubocop-govuk
-    allowed_semver_bumps:
-      - patch
-      - minor
+api_version: 2
+defaults:
+  allowed_semver_bumps:
+    - patch
+    - minor
+  auto_merge: true
+  update_external_dependencies: true