fix: server in shutdown should return 503 instead of 403 (#13496)

various situations where the client is retrying the request
server going through shutdown might incorrectly send 403
which is a non-retriable error, this PR allows for clients
when they retry an attempt to go to another healthy pod
or server in a distributed cluster - assuming it is a properly
load-balanced setup.

internal/http/server.go

@@ -90,7 +90,7 @@ func (srv *Server) Start(ctx context.Context) (err error) {
 		if atomic.LoadUint32(&srv.inShutdown) != 0 {
 			// To indicate disable keep-alives
 			w.Header().Set("Connection", "close")
-			w.WriteHeader(http.StatusForbidden)
+			w.WriteHeader(http.StatusServiceUnavailable)
 			w.Write([]byte(http.ErrServerClosed.Error()))
 			w.(http.Flusher).Flush()
 			return