Use V2 of GOV.UK Dependabot Merger

GOV.UK Dependency Merger configuration changed to allow automatic patching of
all dependencies as described in RFC-167 [^1]. Consequently V1 no longer works.
This updates the configuration to use the version 2 convention to reinstate the
basic functionality of automatic patching of internal dependencies only.
The default allowed_semver_bumps are [patch, minor].

Follow up work will be required to enable automatic patching of external
dependencies, which we encourage the team to do.

[^1]: https://github.com/alphagov/govuk-rfcs/blob/main/rfc-167-auto-patch-dependencies.md

.govuk_dependabot_merger.yml

@@ -1,49 +1,4 @@
-api_version: 1
-auto_merge:
-  - dependency: gds-api-adapters
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govspeak
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_ab_testing
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_app_config
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_personalisation
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_publishing_components
-    allowed_semver_bumps:
-      - patch
-  - dependency: plek
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: rails_translation_manager
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: slimmer
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_schemas
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: govuk_test
-    allowed_semver_bumps:
-      - patch
-      - minor
-  - dependency: rubocop-govuk
-    allowed_semver_bumps:
-      - patch
-      - minor
+api_version: 2
+defaults:
+  auto_merge: true
+  update_external_dependencies: false # TODO: enable after verifying that this repo meets the conditions