Configure automatic merging for internal deps

This is the first step in configuring automatic merging for all the
internal (GOV.UK) dependencies, per documentation[1].

Only patch and minor versions will be eligible.

The next step will be adding signon to the opted-in repos [2], but it is
contingent on getting this config merged.

[1] https://github.com/alphagov/govuk-dependabot-merger?tab=readme-ov-file#usage
[2] https://github.com/alphagov/govuk-dependabot-merger/blob/main/config/repos_opted_in.yml

app/helpers/components/.govuk_dependabot_merger.yml

@@ -0,0 +1,30 @@
+api_version: 1
+auto_merge:
+  - dependency: gds-api-adapters
+    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_sidekiq
+    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