Permissions gcp secret manager

What does this MR do and why?

This MR updates the documentation for IAM permissions related to GCP secret manager. The documentation suggests to grant the GitLab project access to the entire GCP project within IAM (therefore all secrets in it) and optionally fine tune this access to specific secrets using IAM conditions. A more secure approach is to give permissions/grant access directly at the secret level and not at the project level.

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