Merge pull request #515 from alphagov/update-dependabot-merger-v2

Upgrade govuk_dependabot_merger config to use v2

.govuk_dependabot_merger.yml

@@ -1,14 +1,7 @@
-api_version: 1
-auto_merge:
-  - dependency: optic14n
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_app_config
-    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