Revise file storage rules

[MrCsabaToth]

Currently the file storage rules are too relaxed because I was getting 403. Looks like @MaggieThomann ran into similar issues: firebase/flutterfire#13530

1. Execute curl -X POST -H "Authorization: Bearer $(gcloud auth print-access-token)" -H "Content-Type: application/json" https://us-central1-aiplatform.googleapis.com/v1/projects/${customer project}/locations/us-central1/endpoints -d '' as [firebase_vertexai]: generateContent doesn't work when file_data is included firebase/flutterfire#13530 (comment)
2. Upgrade to latest firebase_vertexai, possibly even with some migration (?)