docs: [AlloyDb] Clarify that [initial_user](http://_vscodecontentref_/3) is not required in Cluster (#8789)

* docs: Clarify that [initial_user](http://_vscodecontentref_/3) is not required in Cluster

The comment for the [initial_user](http://_vscodecontentref_/4) field in the [Cluster](http://_vscodecontentref_/5) 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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