Clarify GCS bucket permission requirements in SFT and RL tutorials

[shralex]

Updates the SFT and RL multi-host tutorials to recommend either the broader Storage Admin role, or combining the restrictive Storage Object Admin role with Storage Legacy Bucket Reader. This ensures JAX/TensorStore can perform bucket-level operations (like storage.buckets.get) and prevents misleading 'bucket not found' errors.

FIXES: b/520780283

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