Examples should move to gateway api since Nginx Ingress is deprecated

[RiRa12621]

At

kube-prometheus/docs/customizations/exposing-prometheus-alertmanager-grafana-ingress.md

Line 66 in be3a410

'nginx.ingress.kubernetes.io/auth-realm': 'Authentication Required',

and other places, the examples reference nginx ingress and customizations to it via annotations. Since Nginx Ingress is deprecated and support with will be halted in March, those examples should be migrated to either another ingress or to Gateway API.
Gut-feeling wise I'd say the more supported and recommended way will be Gateway API so that makes the most sense

ref for nginx deprecation: https://kubernetes.io/blog/2025/11/11/ingress-nginx-retirement/