Add workflow to enforce gem version bumps

This workflow will run when opening a pull request on gem repositories.
It checks if there are changes to the CHANGELOG.md and version.rb/*.gemspec files.
If those files have not been changed the workflow will fail.

Code changes are often merged into gems without the gem version being bumped.
This leads to changes not being released quickly and sometimes piling up.
Enforcing all PRs include a gem version bump will eliminate the problem and allow us
to remove all the checks and alerts we have for unreleased gem changes.

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

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

@@ -0,0 +1,105 @@
+name: Gem Bump Checker
+
+on:
+  workflow_call:
+    inputs:
+      gem_name:
+        required: false
+        type: string
+        default: ${{ github.event.repository.name }}
+    secrets:
+      GH_TOKEN:
+        required: true
+
+jobs:
+  check-files:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+      with:
+        fetch-depth: 0
+        show-progress: false
+        token: ${{ secrets.GH_TOKEN }}
+    - uses: ruby/setup-ruby@v1
+      with:
+        rubygems: latest
+        bundler-cache: true
+
+    - name: "Determine HEAD of default branch"
+      id: fetch_default_branch_head
+      run: echo "sha=$(git ls-remote origin HEAD | cut -f 1)" >> "$GITHUB_OUTPUT"
+
+    - name: "Determine HEAD of current branch"
+      id: fetch_current_branch_head
+      run: echo "sha=$(git rev-parse HEAD)" >> "$GITHUB_OUTPUT"
+
+    - name: Check PR author
+      id: check_pr_author
+      run: echo "pr_author=${{ github.event.pull_request.user.login }}" >> "$GITHUB_OUTPUT"
+
+    - name: Check for version and changelog updates
+      if: steps.check_pr_author.outputs.pr_author != 'dependabot[bot]'
+      env:
+        GH_TOKEN: ${{ secrets.GH_TOKEN }}
+        BASE_REF: ${{ steps.fetch_default_branch_head.outputs.sha }}
+        HEAD_REF: ${{ steps.fetch_current_branch_head.outputs.sha }}
+      run: |
+        MODIFIED_FILES=$(git diff --name-only "$BASE_REF".."$HEAD_REF")
+
+        if ! echo "$MODIFIED_FILES" | grep 'version.rb' && \
+           ! echo "$MODIFIED_FILES" | grep '\.gemspec$'; then
+          echo "Error: Either version.rb or a .gemspec file must be modified with each pull request to bump the gem version."
+          exit 1
+        fi
+
+        if ! echo "$MODIFIED_FILES" | grep 'CHANGELOG.md'; then
+          echo "Error: CHANGELOG.md must be updated with each pull request."
+          exit 1
+        fi
+
+    - name: Dependabot version bump
+      if: steps.check_pr_author.outputs.pr_author == 'dependabot[bot]'
+      env:
+        GEM_NAME: ${{ inputs.gem_name }}
+        GH_TOKEN: ${{ secrets.GH_TOKEN }}
+        BRANCH_NAME: ${{ github.event.pull_request.head.ref }}
+      run: |
+        set -euo pipefail
+
+        git checkout "$BRANCH_NAME"
+
+        curr_ver=$(ruby -r rubygems -e "puts Gem::Specification::load('$GEM_NAME.gemspec').version")
+        major=$(echo "$curr_ver" | cut -d . -f1)
+        minor=$(echo "$curr_ver" | cut -d . -f2)
+        patch=$(echo "$curr_ver" | cut -d . -f3)
+        next_ver="$major.$minor.$((patch + 1))" # Increment the patch version
+
+        if ! [[ -r CHANGELOG.md ]]; then
+          echo "CHANGELOG.md not found or not readable. Exiting."
+          exit 0
+        fi
+
+        if ! grep -qR --include "version.rb" --include "*.gemspec" "$curr_ver"; then
+          echo "Can't find version.rb or *.gemspec containing the string '$curr_ver'. Exiting."
+          exit 0
+        fi
+
+        find . -type f \( -iname "version.rb" -o -iname "*.gemspec" \) -exec sed -i "0,/$curr_ver/s//${next_ver}/" {} \;
+
+        printf "# %s\n\n* Update dependencies\n\n" "$next_ver" > NEW_CHANGELOG.md
+        cat CHANGELOG.md >> NEW_CHANGELOG.md
+        mv NEW_CHANGELOG.md CHANGELOG.md
+
+        git config --global user.name "GOV.UK Continuous Integration User"
+        git config --global user.email "[email protected]"
+
+        git add CHANGELOG.md
+
+        find . \( -path ./vendor/bundle -prune \) -o \( -type f \( -iname version.rb -o -iname "*.gemspec" \) -print \) | while read -r file; do
+          file_path=${file#./}
+          git add "$file_path"
+        done
+
+        git commit -m "Automated version bump to v$next_ver"
+        git push origin "$BRANCH_NAME"