fix(docs): bad example for oidc setup with gcp

[hamir-suspect]

📝 Description

The previous Google Cloud OIDC configuration had several problems:

1. Circular configuration: Set google.subject to a static value, then checked if it equals that same static value
2. Length limitation: Semaphore's JWT subject often exceeds Google Cloud's 127-byte limit
3. No actual access control: The condition would always be true, defeating the security purpose

✅ Checklist

• I have tested this change
• This change requires documentation update