ci(backport): add backport action & releases (nelmio#2594)

## Description

Implements https://github.com/korthout/backport-action to create a
backport

## What type of PR is this? (check all applicable)
- [ ] Bug Fix
- [ ] Feature
- [ ] Refactor
- [ ] Deprecation
- [ ] Breaking Change
- [ ] Documentation Update
- [x] CI

## Checklist
- [ ] I have made corresponding changes to the documentation (`docs/`)
- [ ] I have made corresponding changes to the changelog
(`CHANGELOG.md`)

Author: DjordyKoert

.github/workflows/backport.yml

@@ -0,0 +1,40 @@
+name: Backport merged pull request
+
+on:
+  pull_request_target:
+    types:
+      - closed
+      - labeled
+
+permissions:
+  contents: write # so it can comment
+  pull-requests: write # so it can create pull requests
+
+jobs:
+  backport:
+    name: Backport pull request
+    runs-on: ubuntu-latest
+    if: >
+      (
+        github.event.action == 'closed' &&
+        github.event.pull_request.merged
+      ) ||
+      (
+        github.event.action == 'labeled'
+        && startsWith(github.event.label.name, 'backport')
+        && github.event.pull_request.merged
+      )
+    steps:
+      - uses: actions/checkout@v4
+      - name: Create backport pull requests
+        uses: korthout/backport-action@v3
+        with:
+          branch_name: backport-#${pull_number}-to-${target_branch}
+          pull_description: |
+            ### This is an automated backport of #${pull_number} to branch `${target_branch}`.
+
+            > [!CAUTION]
+            > **Do not modify this pull request.**
+            
+            ${pull_description}
+          pull_title: "${pull_title} [Backport #${pull_number} to ${target_branch}]"

.github/workflows/release.backport.yml

@@ -22,4 +22,4 @@ jobs:
           # for more options
           release-type: php
           versioning-strategy: always-bump-patch
-          release-as: 4.0.0
+          target-branch: 4.x