docs: Clarify that initial_user is not required in Cluster (#32253)

* docs: Clarify that initial_user is not required in Cluster

The comment for the initial_user field in the Cluster message has been updated to remove the "Required." annotation, better reflecting its optional nature.

PiperOrigin-RevId: 841859434

Source-Link: googleapis/googleapis@35365cb

Source-Link: googleapis/googleapis-gen@18ed7a9
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google-cloud-alloy_db-v1/proto_docs/google/cloud/alloydb/v1/resources.rb

@@ -480,7 +480,7 @@ class MaintenanceSchedule
         #     system actions like failover or maintenance.
         # @!attribute [rw] initial_user
         #   @return [::Google::Cloud::AlloyDB::V1::UserPassword]
-        #     Input only. Initial user to setup during cluster creation. Required.
+        #     Input only. Initial user to setup during cluster creation.
         #     If used in `RestoreCluster` this is ignored.
         # @!attribute [rw] automated_backup_policy
         #   @return [::Google::Cloud::AlloyDB::V1::AutomatedBackupPolicy]