Workflows: try a backport changelog (WordPress#61785)

Co-authored-by: ellatrix <ellatrix@git.wordpress.org>
Co-authored-by: priethor <priethor@git.wordpress.org>
Co-authored-by: vcanales <vcanales@git.wordpress.org>

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

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+name: Verify Core Backport Changlog
+
+on:
+    pull_request:
+        types: [opened, synchronize]
+        paths:
+            - 'lib/**'
+            - '!lib/load.php'
+            - '!lib/experiments-page.php'
+            - '!lib/experimental/**'
+            - 'phpunit/**'
+            - '!phpunit/experimental/**'
+            - '!phpunit/blocks/**'
+            - 'packages/**/*.php'
+            - '!packages/block-library/**'
+            - '!packages/e2e-tests/**'
+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: ${{ github.event.number }}
+              run: |
+                  changelog_folder="backport-changelog"
+
+                  # Find any changelog file that contains the Gutenberg PR link
+                  gutenberg_pr_url="https://github\.com/WordPress/gutenberg/pull/${PR_NUMBER}"
+                  changelog_file=$(grep -rl "[-*] ${gutenberg_pr_url}" "${changelog_folder}" | head -n 1)
+
+                  # Confirm that there is an entry containing the Gutenberg PR link
+                  if [[ -z "${changelog_file}" ]]; then
+                    echo "Please create a core backport PR and add a file with the path <wp-release-number>/<core-pr-number>.md in the $changelog_folder folder with the core backport PR URL and a list item with this PR URL."
+                    echo "If changes are related to an existing, open core PR, you may add this PR URL to the core PR's file."
+                    echo "See $changelog_folder/readme.md for more information."
+                    exit 1
+                  fi
+
+                  core_pr_number=$(basename "${changelog_file}" .md)
+                  core_pr_url="https://github\.com/WordPress/wordpress-develop/pull/${core_pr_number}"
+
+                  # Confirm that the entry has the correct core backport PR URL.
+                  if ! grep -q -e "${core_pr_url}" "${changelog_file}"; then
+                    echo "Please update the content of ${changelog_file} to include the core backport PR URL, or update the file name to match the core backport PR number."
+                    exit 1
+                  fi

backport-changelog/readme.md

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+# Core Backport Changelog
+
+Any PR that makes changes to be backported to [core](https://github.com/WordPress/wordpress-develop) should log a core PR here. It's possible to have multiple Gutenberg PRs link to a single core backport PR. The core backport PR can remain open as long as wanted/needed. The entries are sorted by core release (in folders), and each entry should be an md file with the core PR number as the file name, and the link to the Gutenberg PR in the file content. The file content should start with the core PR URL, followed by a Markdown list of Gutenberg PRs (see example). Files are used to avoid rebase conflicts.
+
+If you think a file path is wrongly flagged as needing a core backport PR, you can add it to the list of exceptions in `.github/workflows/check-backport-changelog.yml`.
+
+## Example
+
+Path: `{wp-release-number-x.x}/{core-pr-number}.md`, e.g. `6.6/1234.md`.
+File content:
+```md
+https://github.com/WordPress/wordpress-develop/pull/{core-pr-number}
+
+* https://github.com/WordPress/gutenberg/pull/{first-gb-pr-number}
+* https://github.com/WordPress/gutenberg/pull/{second-gb-pr-number}
+```