Automatic merge from submit-queue (batch tested with PRs 57896, 58070). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
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.
```
23226c24d4