GitHub: Migrate Repo Lockdown to an action.

Repo Lockdown started out as a GitHub app[1], but has since been
migrated to an action[2]. Update our config to match. Fixes #17692.

[1] https://github.com/apps/repo-lockdown
[2] https://github.com/dessant/repo-lockdown

.github/lockdown.yml

@@ -1,10 +1,26 @@
 # Configure the Repo Lockdown GitHub App to automatically close pull requests.
-# See https://github.com/apps/repo-lockdown
+# See https://github.com/dessant/repo-lockdown
 
-pulls:
-  comment: |
-    Hi, thank you for your contribution!
+name: 'Repo Lockdown'
 
-    GitHub is however not the right place for these, please have a look at
-    <https://www.wireshark.org/docs/wsdg_html_chunked/ChSrcContribute.html>
-    for further instructions.
+on:
+  pull_request_target:
+    types: opened
+  schedule:
+    - cron: '7 * * * *'
+
+permissions:
+  pull-requests: write
+
+jobs:
+  action:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: dessant/repo-lockdown@v2
+        with:
+          pr-comment: >
+            Hi, thank you for your contribution!
+
+            GitHub is however not the right place for these, please have a look at
+            <https://www.wireshark.org/docs/wsdg_html_chunked/ChSrcContribute.html>
+            for further instructions.