Using raw manifests with kustomize: namespace `envoy-gateway-system` is hardcoded into the topology injector webhook causing a mismatch during patching by certgen install hook

*Description*:
When using the raw manifests as Kustomize resources (with ArgoCD for instance),
the namespace of several resources is hardcoded with a suffix of 'envoy-gateway-system'. This causes problem during the installation because the cert-gen hook try to patch the webhook using a name derived from the namespace, which obviously does not work if you changed the namespace.

*Repro steps*:

Run `kustomize build .` in a directory with the following `kustomization.yaml`, and grep the result for '.envoy-gateway-system':
```yaml
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: api-gateway

resources:
- https://github.com/envoyproxy/gateway/releases/download/v1.7.0/install.yaml
```

>**Note**: If there are privacy concerns, sanitize the data prior to
sharing.

*Environment*:
N/A

*Logs*:

This is from deploying with ARGOCD, in the eg-gateway-helm-certgen Job
```
2026-02-26T09:40:44.515Z INFO cmd/certgen.go:81 generated certificates
2026-02-26T09:40:44.539Z INFO cmd/certgen.go:125 [api-gateway/envoy-gateway api-gateway/envoy api-gateway/envoy-rate-limit api-gateway/envoy-oidc-hmac]: skipped creating secret since it already exists;Either update the secrets manually or set overwriteControlPlaneCerts in the EnvoyGateway config
Error: failed to patch webhook: failed to get mutating webhook configuration: mutatingwebhookconfigurations.admissionregistration.k8s.io "envoy-gateway-topology-injector.api-gateway" not found
Usage:
envoy-gateway certgen [flags]

Flags:
--config-home string Directory for certificates (defaults to ~/.config/envoy-gateway
--disable-topology-injector Disables patching caBundle for injector MutatingWebhookConfiguration.
-h, --help help for certgen
-l, --local Generate all the certificates locally.
-o, --overwrite Updates the secrets containing the control plane certs.

failed to patch webhook: failed to get mutating webhook configuration: mutatingwebhookconfigurations.admissionregistration.k8s.io "envoy-gateway-topology-injector.api-gateway" not found
```

working around this is somewhat easy enough (just patching the webhook name and target namespace), but it's probably worth mentioning.