Add gem version bump checker workflow

This workflow will check every pull request and fail if it does not make changes to the changelog and the version.rb or .gemspec file.

This will not be a required check to start with.

https://trello.com/c/R1kkdVjt/3560-re-evaluate-effectiveness-of-gem-auto-release-workflow-3

.github/workflows/gem-bump-checker.yml

@@ -0,0 +1,9 @@
+on:
+  workflow_dispatch: {}
+  pull_request: {}
+
+jobs:
+  gem-bump-checker:
+    uses: alphagov/govuk-infrastructure/.github/workflows/gem-bump-checker.yml@main
+    secrets:
+      GH_TOKEN: ${{ secrets.GOVUK_CI_GITHUB_API_TOKEN }}