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>

.github/labeler.yml

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

.github/workflows/label-pr.yml

@@ -0,0 +1,15 @@
+name: Label PRs
+on:
+  pull_request_target:
+    types: [opened]
+permissions: {}
+jobs:
+  label:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/labeler@8558fd74291d67161a8a78ce36a881fa63b766a9 # pin@v5.0.0
+        with:
+          sync-labels: true