gha(stale): Add Pull Request to stale target

Author: peaceiris

.github/workflows/stale.yml

@@ -1,4 +1,4 @@
-name: "Close stale issues"
+name: "Stale"
 
 on:
   schedule:
@@ -12,5 +12,6 @@ jobs:
       with:
         repo-token: ${{ secrets.GITHUB_TOKEN }}
         stale-issue-message: 'This issue is stale because it has been open 21 days with no activity. Remove stale label or comment or this will be closed in 7 days.'
+        stale-pr-message: 'This pull request is stale because it has been open 21 days with no activity. Remove stale label or comment or this will be closed in 7 days.'
         days-before-stale: 21
         days-before-close: 7