Do not allow to override kingress's ingress.class via ksvc's annotation

## In what area(s)?

/area API
/area networking

## Describe the feature

Currently `ingress.class` in Kingress is overriden via annotation as the following steps:

### 1. Add ingress-class annotating to ksvc

```
$ oc annotate ksvc hello-example networking.knative.dev/ingress-class=foo
```
or
```
$ oc annotate ksvc hello-example networking.knative.dev/ingress.class=foo
```

### 2. the annotation in Kingress is overriden.

```
$ kubectl get king -o yaml |grep ingress.class
networking.knative.dev/ingress.class: foo
```

This causes a problem - for example, a managed service supports Kourier only (users cannot modify `config-network`) but they try to customize it via annotation and cause a problem.
Hence, we would like to disallow to override the annotation. i.e. do not propagate the ingress class annotation into kingress from ksvc's annotation.

### Side question

Is there use-case for the annotation override? If somebody uses multiple Ingress on the same cluster, we should not change this...

Start by tracing how the ksvc ingress-class annotations reach Kingress, focusing on the Kingress and ksvc paths described in the issue. Confirm the expected behavior for both annotation names and the config-network setting, then verify that the ingress class is no longer propagated from ksvc annotations without changing supported multi-ingress use cases.

Written by the indexing model from the issue text.