Docs: update SAML IdP GCP workforce guide with IAM role prerequisite and new input label copy

[flyinghermit]

https://github.com/gravitational/teleport.e/pull/4396 updated SAML GCP workforce integration UI with a prerequisites mentioning required roles and updated new input label. This PR updates the GCP workforce guide to reflect those latest changes.

Part of - https://github.com/gravitational/teleport/issues/40927

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