Fix conformance for GatewayHTTPSListenerDetectMisdirectedRequests

We should return 421 when http/2 connection coalescing is detected

> one workaround for now, is to disable http/2 ALPN when overlapping hostnames are detected, with the `OverlappingTLSConfig` condition added to the listeners https://gateway-api.sigs.k8s.io/geps/gep-3567/
>
> I might be technically not deep enough into this https://gateway-api.sigs.k8s.io/geps/gep-3567/ states an option b of returning 421 for this. Is there a reason that envoy gateway is doing that which i am missing and/or could this be tracked as a followup in this or a new issue? Ideally it would be nice to not break http2 support like this :)

_Originally posted by @MTRNord in [#2675](https://github.com/envoyproxy/gateway/issues/2675#issuecomment-2841179443)_