Gardening: add new Flag OSS action (#117)

This allows adding a label and posting a Slack message every time someone opens a PR from a fork.

See #19382

Internal reference: p3btAN-1nR-p2#comment-13830

.github/workflows/gardening.yml

@@ -1,6 +1,9 @@
 name: Repo gardening
 
 on:
+  # Listen to this event for PRs from forks (see Flag OSS task).
+  pull_request_target:
+    types: [opened]
   # We need to listen to all these events to catch all scenarios
   pull_request:
     types: ['opened', 'synchronize', 'edited', 'closed', 'labeled']
@@ -26,10 +29,11 @@ jobs:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
      - name: 'Run gardening action'
-       uses: automattic/action-repo-gardening@v1.1.0
+       uses: automattic/action-repo-gardening@v1.2.0
        with:
          github_token: ${{ secrets.GITHUB_TOKEN }}
          slack_token: ${{ secrets.SLACK_TOKEN }}
          slack_design_channel: ${{ secrets.SLACK_DESIGN_CHANNEL }}
          slack_editorial_channel: ${{ secrets.SLACK_EDITORIAL_CHANNEL }}
-         tasks: 'assignIssues,cleanLabels,notifyDesign,notifyEditorial'
+         slack_team_channel: ${{ secrets.SLACK_TEAM_CHANNEL }}
+         tasks: 'assignIssues,cleanLabels,notifyDesign,notifyEditorial,flagOss'