ci: relay fork PR reviews to giteabot through workflow_run (#39546)

`pull_request_review` runs on fork PRs, which includes all backport PRs,
get a read-only token and no secrets, so giteabot cannot write lgtm
labels and statuses there. A no-op `giteabot-review` workflow now
triggers giteabot through `workflow_run`, which gets both. This allows
retiring the legacy fly.io webhook bot.

Part of https://github.com/go-gitea/giteabot/issues/15

.github/workflows/giteabot-backport.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 30
     steps:
-      - uses: go-gitea/giteabot@4c9d4d3fd913b5c35f59dcc0b004a3d11d5b22bf # v1.0.7
+      - uses: go-gitea/giteabot@f48c6a15e0d384f037aea19cc05ff5e9551096b9 # v1.1.0
         with:
           github_token: ${{ secrets.GITEABOT_TOKEN }}
           gitea_fork: giteabot/gitea