Issue title: HNC: CRD metadata.name validation for singletons
 Issue body: Add kustomization patch to add metadata.name validation for singletons. With this, we can remove the validating webhooks on the singleton names and remove the `CritSingletonNameInvalid`  HNCConfiguration condition.

HNCConfiguration CRD:
```
CRD.spec.validation.openAPIV3Schema.properties.
metadata:
  type: object
  properties:
    name:
      type: string
      pattern: "^config$"
```

HierarchyConfiguration CRD:
```
CRD.spec.validation.openAPIV3Schema.properties.
metadata:
  type: object
  properties:
    name:
      type: string
      pattern: "^hierarchy$"
```

/assign @adrianludwin 
 Comments: 
 Comment 0: /assign @yiqigao217 
/unassign @adrianludwin 
{'source': 'GithubIssues', 'repo_name_with_owner': 'kubernetes-sigs/multi-tenancy', 'issue_id': 647708522, 'repo_stars': 898.0}