Remove the deprecated webhook interface usage of the controller runtime

[yanmxa]

The controller runtime removed the deprecated webhook interface in v0.20.x, but this repository still appears to contain these interfaces. It is recommended to update the controller runtime package and removing these interfaces to avoid the following error:

Error: 78.98 /go/pkg/mod/github.com/stolostron/multiclusterhub-operator@v0.0.0-20250415191038-1e368a726d8b/api/v1/multiclusterhub_webhook.go:102:15: undefined: webhook.Defaulter
Error: 78.98 /go/pkg/mod/github.com/stolostron/multiclusterhub-operator@v0.0.0-20250415191038-1e368a726d8b/api/v1/multiclusterhub_webhook.go:111:15: undefined: webhook.Validator

Reference:

• Remove deprecated webhook interfaces kubernetes-sigs/controller-runtime#2641
• ⚠ Remove deprecated Defaulter and Validator interfaces kubernetes-sigs/controller-runtime#2877
• Changes to webhook.Validator pattern cause version update problems kubernetes-sigs/controller-runtime#2596