ci: auto-label PRs targeting main with v4.x

Mirrors the dev-v3 labeling workflow from #32340, reusing the
existing v4.x label since main tracks the upcoming v4 release.

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

.github/labeler.yml

@@ -0,0 +1,2 @@
+"v4.x":
+  - base-branch: "main"

.github/workflows/label-pr.yml

@@ -0,0 +1,16 @@
+name: Label PRs
+on:
+  pull_request_target:
+    branches: [main]
+    types: [opened, reopened, edited, synchronize]
+permissions: {}
+jobs:
+  label:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/labeler@b8dd2d9be0f68b860e7dae5dae7d772984eacd6d # pin@v6.2.0
+        with:
+          sync-labels: true