infra: Convert from old autolabeler GitHub App to new GitHub actions version.

The old autolabeler GitHub App is no longer supported and broken. The
new GitHub actions autolabeler is maintained by GitHub.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

.github/workflows/labeler.yml

@@ -0,0 +1,11 @@
+name: "Pull Request Labeler"
+on:
+- pull_request_target
+
+jobs:
+  triage:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/labeler@main
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"