ci: migrate to webhook-based stale bot

While at it, we synchronized .github/formalities.json with the current
default settings of the webhook bot.

Since the bot is shared across all OpenWrt repositories, the stale PR
cleanup is disabled by default at the repository level. Because this
repository wants to use stale triage, we explicitly enable it here.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>

.github/formalities.json

@@ -19,5 +19,8 @@
   "branch_labeling": true,
   "check_openwrt_meta": true,
   "check_conffiles": true,
-  "check_patch_headers": true
+  "check_patch_headers": true,
+  "check_pkg_release": "warning",
+  "require_linked_github_account": true,
+  "enable_stale_bot": true
 }

.github/workflows/stale-bot.yml

@@ -1,30 +0,0 @@
-name: Mark and close stale PRs
-
-on:
-  schedule:
-    # Run once a day at 05:30
-    - cron: '30 5 * * *'
-
-permissions:
-  actions: write
-  # BUG: required due to a bug in the stale action:
-  #      https://github.com/actions/stale/issues/840#issuecomment-2195082494
-  issues: read
-  pull-requests: write
-
-jobs:
-  stale:
-    runs-on: ubuntu-slim
-    steps:
-      - name: Mark and close PRs with formality issues
-        uses: actions/stale@v10
-        with:
-          repo-token: ${{ secrets.GITHUB_TOKEN }}
-          stale-pr-message: |
-            This PR is stale because it has been open for 14 days with no activity and has the "not following guidelines" label.
-            It will be closed if no further activity occurs within 14 days.
-          close-pr-message: 'This PR was closed because it has been marked stale for 14 days with no activity.'
-          days-before-stale: 14
-          days-before-close: 14
-          only-labels: 'not following guidelines'
-          stale-pr-label: 'stale'