ci: auto-label PRs targeting dev-v3 (#32340)

* ci: auto-label PRs targeting dev-v3

Use actions/labeler to apply a "dev-v3" label on PRs
opened against the `dev-v3` branch.

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

* ci: bump actions/labeler to v6.2.0

https://github.com/actions/labeler/releases/tag/v6.2.0

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

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Benoit Tigeot <benoittgt@users.noreply.github.com>
Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>

* ci: reuse existing v3.x label for dev-v3 PRs

Avoid creating a new label since v3.x already exists in the repo

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

---------

Signed-off-by: Benoit Tigeot <benoit.tigeot@lifen.fr>
Signed-off-by: Benoit Tigeot <benoittgt@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

.github/labeler.yml

@@ -0,0 +1,2 @@
+"v3.x":
+  - base-branch: "dev-v3"

.github/workflows/label-pr.yml

@@ -0,0 +1,16 @@
+name: Label PRs
+on:
+  pull_request_target:
+    branches: [dev-v3]
+    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