Describe the bug:
When using the ACME HTTP-01 solver with Gateway API and requesting a certificate for an IP address, cert-manager sets HTTPRoute.spec.hostnames[0] to that IP. For IPv6 this is immediately rejected by the API server, for example:

The HTTPRoute "httproute-test" is invalid: spec.hostnames[0]: Invalid value: "2001:db8::1": spec.hostnames[0] in body should match ^(\*\.)?[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*$

Even if IPv4 (e.g. 192.0.2.1) can sometimes appear to work, the Gateway API spec explicitly says that hostname does not support IP literals: https://gateway-api.sigs.k8s.io/reference/spec/#hostname

The hostname field on HTTPRoute is optional, so for IP challenges it should be left unset.

Expected behaviour:
For HTTP-01 challenges where the target is an IP address (IPv4 or IPv6), cert-manager should not set HTTPRoute.spec.hostnames at all. The solver should still be able to complete HTTP-01 validation using a hostname-less HTTPRoute. For normal DNS names, the current behaviour (setting spec.hostnames to the DNS name) should remain unchanged.

Steps to reproduce the bug:

1. Install cert-manager with Gateway HTTP-01 enabled.
2. Create a Certificate that targets an IP address, for example:

   apiVersion: cert-manager.io/v1
   kind: Certificate
   metadata:
     name: ip-cert
   spec:
     ipAddresses:
       - 2001:db8::1
     issuerRef:
       name: letsencrypt-http01
       kind: ClusterIssuer
       group: cert-manager.io
     secretName: ip-cert