github: convert pull closing to GH actions

The probot app we were using has been shutdown, so switch over to
the new GH actions flow.

Change-Id: Ifa8c2835e1ac1a4df53a5c4f0aa851fbacbd4096
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/3217681
Reviewed-by: Mark Mentovai <mark@chromium.org>

.github/workflows/close-pull-request.yml

@@ -0,0 +1,22 @@
+# GitHub actions workflow.
+# https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions
+
+# https://github.com/superbrothers/close-pull-request
+name: Close Pull Request
+
+on:
+  pull_request_target:
+    types: [opened]
+
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: superbrothers/close-pull-request@v3
+      with:
+        comment: >
+          Thanks for your contribution!
+          Unfortunately, we don't use GitHub pull requests to manage code
+          contributions to this repository.
+          Instead, please see [README.md](../blob/HEAD/README.md) which
+          provides full instructions on how to get involved.