Add automerge and autoupdate labels to backport PRs #1307

Backport PRs opened by dapr-bot don't consistently carry the automerge and autoupdate labels. Some release-1.19 backports have them because someone added them by hand (#1268, #1270, #1272), and others have none (#1288, #1290, #1291, #1292, #1293, #1294, #1295), so they wait until a maintainer notices and labels them.

The shared backport workflow in dapr/.github already supports this through its labels-template input, which defaults to []. Our caller in .github/workflows/backport.yaml doesn't set it. The fix is to pass it:

jobs:
  backport:
    uses: dapr/.github/.github/workflows/backport.yaml@main
    with:
      labels-template: '["automerge", "autoupdate"]'
    secrets:
      dapr_bot_token: ${{ secrets.DAPR_BOT_TOKEN }}

The labels are only applied to backports that cherry-pick cleanly. When the cherry-pick conflicts, the bot opens a placeholder PR with a failing backport/conflict status and no labels, and whoever resolves it adds them. That's fine and out of scope here.

To verify after merging, add backport release-1.19 to a merged PR and check that the backport PR the bot opens has both labels.