KKP 2.30 - Update Installation instruction to gateway API

[toschneck]

As ingress is depricated installation should contain how to config via gateway api:

changelog 2.30:

Kubermatic now supports Gateway API for external traffic routing as an alternative to NGINX Ingress. It can be enabled via the --enable-gateway-api operator flag (set via migrateGatewayAPI: true in Helm values and --migrate-gateway-api in kubermatic-installer) (#15293)

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