Workflows: try a backport changelog

.github/workflows/check-backport-changelog.yml

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+name: Verify Core Backport Changlog
+
+on:
+    pull_request:
+        types: [opened, synchronize]
+        paths:
+            - 'lib/**'
+            - 'packages/**/*.php'
+            - 'phpunit/**'
+jobs:
+    check:
+        name: Check CHANGELOG diff
+        runs-on: ubuntu-latest
+        steps:
+            - name: 'Get PR commit count'
+              run: echo "PR_COMMIT_COUNT=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+            - name: Checkout code
+              uses: actions/checkout@44c2b7a8a4ea60a981eaca3cf939b5f4305c123b # v4.1.5
+              with:
+                  ref: ${{ github.event.pull_request.head.ref }}
+                  repository: ${{ github.event.pull_request.head.repo.full_name }}
+                  fetch-depth: ${{ env.PR_COMMIT_COUNT }}
+                  show-progress: ${{ runner.debug == '1' && 'true' || 'false' }}
+            - name: 'Fetch relevant history from origin'
+              run: git fetch origin ${{ github.event.pull_request.base.ref }}
+            - name: Check CHANGELOG status
+              env:
+                  PR_NUMBER: ${{ github.event.number }}
+              run: |
+                  changelog_path="backport-changelog.md"
+
+                  # Fail if the PR doesn't touch the changelog
+                  if git diff --quiet ${{ github.event.pull_request.base.sha }} HEAD -- "$changelog_path"; then
+                    echo "Please add a CORE PR LINK (in Markdown) to $changelog_path"
+                    echo "If changes are related to an existing, open PR, you may use that PR"
+                    exit 1
+                  fi
+
+                  pr_link_grep_pattern="https://github\.com/WordPress/gutenberg/pull/\([0-9]\+\) for https://github\.com/WordPress/gutenberg/pull/${PR_NUMBER}"
+
+                  # Confirm that the CHANGELOG has an entry for the current PR
+                  if ! grep -nq -e "${pr_link_grep_pattern}" "${changelog_path}"; then
+                      echo "Please add a CORE PR LINK to $changelog_path. Make sure it is linked in Markdown."
+                      echo "If changes are related to an existing, open PR, you may use that PR"
+                      exit 1
+                  fi

backport-changelog.md

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+# Core Backport Changelog
+
+Any PR that makes changes to be backported should log a core PR here.
+
+## 6.6