Hardcode kgateway service name

[jenshu]

Since our controller makes an assumption about the control plane service being named kgateway we should hardcode this in the helm chart for now.

Also removed "gloo" references from some constants.

Fixes #10659

[lgadban]

should we remove the fullnameOverride value for now?

[yuval-k]

can we fix this the other way around?
i.e. add the service name as an env-var in the deployment, and use that for the xds host?

[yuval-k]

for reference, what i'm suggesting is to add an env var in the kgateway deployment:

name: XDS_HOST
value: {{ include "kgateway.fullname" . }}

and use that in GetControlPlaneXdsHost

func GetControlPlaneXdsHost() string {
	return kubeutils.ServiceFQDN(metav1.ObjectMeta{
		Name:      os.Getenv("XDS_HOST"),
		Namespace: namespaces.GetPodNamespace(),
	})
}

We may want to tweak this, so we centralize our processing of env-vars.

[lgadban]

Adding on to @yuval-k 's last point, the existing "settings" processing is centralized here currently: https://github.com/kgateway-dev/kgateway/blob/main/internal/kgateway/extensions2/settings/settings.go#L7-L12

This was intended to be as lightweight as possible while still providing a standardized approach.
IMO this should be sufficient for the use case we want to accomplish in this PR.

In general though, we may want to reevaluate this approach given that the controller binary is now ran via cobra. Viper is fairly idiomatic and allows to define configuration in a single place and demotes the env-var specifics to an implementation detail.