Better distinguish the two kinds of proxy health check servers

Kube-proxy runs two different health servers; one for monitoring the
health of kube-proxy itself, and one for monitoring the health of
specific services. Rename them to "ProxierHealthServer" and
"ServiceHealthServer" to make this clearer, and do a bit of API
cleanup too.

cmd/kube-proxy/app/server.go

@@ -482,7 +482,7 @@ type ProxyServer struct {
 	UseEndpointSlices      bool
 	OOMScoreAdj            *int32
 	ConfigSyncPeriod       time.Duration
-	HealthzServer          *healthcheck.HealthzServer
+	HealthzServer          *healthcheck.ProxierHealthServer
 }
 
 // createClients creates a kube client and an event client from the given config and masterOverride.