feat: remove legacy app engine deployment artifacts

This commit removes obsolete files and configurations related to the
previous App Engine deployment of the internal-launchpad service.
These artifacts are no longer needed since the service has been
migrated to Cloud Run.

The following changes were made:

- Deleted `fix-appengine-permissions-complete.sh`,
 `fix-project-level-permissions.sh`, and
 `create-missing-repository.sh` scripts.
- Removed the `app.yaml` file from `firebase/internal-launchpad/`.
- Updated `firebase.json` to remove the `launchpad` hosting target.
- Added `security` label to `labels.yml`

labels.yml

@@ -20,6 +20,9 @@
 - name: "refactor"
   description: "Code changes that neither fix a bug nor add a feature"
   color: "f29513"
+- name: "security"
+  description: "Tasks related to security enhancements, audits, or fixes"
+  color: "000000" # Black for security, to make it stand out
 
 # By Platform
 - name: "platform:web"