Make backporting action use PRs to support protected branches [DOC-26…

…8] (#1396)

In [DOC-255](https://hazelcast.atlassian.net/browse/DOC-255), branch
protection was added - preventing direct `push`es.

Instead, the backport workflow should create PRs to backport the
changes. This gives the _possibility_ for pre-merge checks to be run
(i.e. preventing introducing dead links etc).

Leverages [_existing_ `backport`
tool](https://github.com/hazelcast/backport), but exposed some issues
which need to be addressed **before this can be merged**:
- hazelcast/backport#14
- hazelcast/backport#15
- hazelcast/backport#16
- hazelcast/backport#17

Tested in a [test repo](https://github.com/JackPGreen2/hz-docs), where a
[dummy PR](JackPGreen2#5) was
[successfully
backported](JackPGreen2#16). However,
due to the complexity of fork-triggering, it wil need to be re-tested
once merged.

Fixes: [DOC-268](https://hazelcast.atlassian.net/browse/DOC-268)

[DOC-255]:
https://hazelcast.atlassian.net/browse/DOC-255?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ
[DOC-268]:
https://hazelcast.atlassian.net/browse/DOC-268?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

.github/workflows/backport-workflow.yml

@@ -23,21 +23,20 @@ jobs:
           labels: ${{ inputs.label-to-check-for }}
 
       - name: See result
+        if: runner.debug == '1'
         run: echo "${{ steps.check_pr_labels.outputs.result }}"
 
       - name: Checkout repository
         if: ${{ steps.check_pr_labels.outputs.result == 'true' }}
         uses: actions/checkout@v4
         with:
+          # ensure the backport target branch is checked out, too
           fetch-depth: 0
 
-      - name: Checkout maintenance branch and cherry-pick
-        if: ${{ steps.check_pr_labels.outputs.result == 'true' }}
-        run: |
-          git config user.name "${GITHUB_ACTOR}"
-          git config user.email "${GITHUB_ACTOR}@users.noreply.github.com"
-
-          git fetch
-          git checkout ${{ inputs.target-branch }}
-          git cherry-pick -x --strategy=recursive -X theirs $GITHUB_SHA
-          git push
+      - if: ${{ steps.check_pr_labels.outputs.result == 'true' }}
+        uses: hazelcast/backport/.github/actions/backport@main
+        with:
+          GITHUB_TOKEN: ${{ github.token }}
+          TARGET_BRANCH: ${{ inputs.target-branch }}
+          REF_TO_BACKPORT: ${{ github.sha }}
+          BACKPORT_OPTIONS: --omit-labels