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Don't remove APIService from apiHandlerManager when its Available Conditions is not True

**What this PR does / why we need it**:

I use my own apiserver works together with `kube-apiserver`, i have a custom resource: `databases` and created a `database` named: `db-name-1`.

When this apiserver is down(for example: OOMKilled), `kubectl get databases db-name-1 -v 10` returns `404 NotFound`:

```
[{
  "metadata": {},
  "status": "Failure",
  "message": "the server could not find the requested resource (get databases.core.example.com db-name-1)”,
  "reason": "NotFound",
  "details": {
    "name": “db-name-1”,
    "group": "core.example.com",
    "kind": “databases”,
    "causes": [
      {
        "reason": "UnexpectedServerResponse",
        "message": "404 page not found"
      }
    ]
  },
  "code": 404
}]
```

But it is not really `NotFound`.

So if the APIService is not available, just return 503.

There was a PR related with this: #57943 

**Release note**:


```release-note
kube-apiserver: requests to endpoints handled by unavailable extension API servers (as indicated by an `Available` condition of `false` in the registered APIService) now return `503` errors instead of `404` errors.
```