remove invalid ingress_gateway_namespace field from kiali cr spec

General MR

Summary

It looks like the ingress_gateway_namespace field was erroneously added to the Kiali CR spec because some older Kiali docs from v2.11 referenced it. Upon investigation, it seems those docs were incorrect, because the field never actually existed in the Kiali CRD spec. The field has been removed from the current docs without mention.

This was found when a customer attempted a manual helm install of the kiali chart using the values generated by the umbrella. They received a schema validation error because the field isn't in the spec. I think we never saw this because we typically rely on a Flux controller to install the chart. The Flux controller defaults to Warn level validation, while a local helm install specifies Strict validation. See these docs for more information about kube api validation levels.

Relevant logs/screenshots

Linked Issue

Closes #3186 (closed)

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