Clarify GCP audit event streaming format

What does this MR do?

This change improves the documentation for setting up Google Cloud Logging integration. Instead of asking users to enter all three pieces of information (project ID, client email, and private key) in a single step, it now breaks this into two separate steps for clarity. More importantly, it adds specific guidance about the private key format - explaining that it should be in PEM format starting with "-----BEGIN PRIVATE KEY-----" and clarifying that users should not upload the entire JSON key file. This helps prevent common configuration mistakes by being more explicit about what format the private key should be in.

Related issues

• https://gitlab.zendesk.com/agent/tickets/656641

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