ci: run test pipeline on merge v0.50 branch (#19582)

.github/workflows/pr_labeler.yml

@@ -12,7 +12,7 @@ jobs:
       pull-requests: write  # for actions/labeler to add labels to PRs
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/labeler@main
+      - uses: actions/labeler@v4 # v5 is broken, ref https://github.com/actions/labeler/issues/712. Do not bump.
         with:
           configuration-path: .github/pr_labeler.yml
           repo-token: "${{ secrets.GITHUB_TOKEN }}"

.github/workflows/test.yml

@@ -4,7 +4,7 @@ on:
   merge_group:
   push:
     branches:
-      - main
+      - release/v0.50.x
 
 permissions:
   contents: read