Deprecate nginx-ingress-controller

### Description of the feature you would like to add / User story

As part of the bigger story https://github.com/kubermatic/kubermatic/issues/15229, we have decided to proceed with Envoy Gateway as the default implementation of gateway-api.

This issue concerns deprecating the nginx-ingress-controller, first marking it as deprecated and documenting it in the next KKP release(2.30), then ultimately removing the component

### Solution details

- [ ] mark nginx-ingress-controller as deprecated
- [ ] add documentation about deprecation

Start by reading the linked parent issue #15229 and locating the Kubermatic documentation for nginx-ingress-controller and the KKP 2.30 release. Confirm how the component should be marked deprecated and add the required deprecation documentation; done means both checklist items are complete.

Written by the indexing model from the issue text.