add gha warning to upstream prs (#11983)

* add gha warning to upstream prs

* update to hashicorp org

Author: megan07

.github/workflows/upstream-mm.yml

@@ -0,0 +1,15 @@
+name: "Upstream To MM"
+on:
+  pull_request_target:
+    types: [opened, ready_for_review, reopened]
+
+jobs:
+  pr-warning:
+    if: ${{ github.actor != 'modular-magician' }}
+    runs-on: ubuntu-latest
+    steps:
+      - name: Post the warning
+        uses: hashicorp/[email protected]
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}
+          warning: "This repository is generated by https://github.com/GoogleCloudPlatform/magic-modules. Any changes made directly to this repository will likely be overwritten. If you have further questions, please feel free to ping your reviewer or, internal employees, reach out to one of the engineers. Thank you!"