fix: remove verbose logging admission-webhook

After disabling HTTP2 connections by default in prometheus-operator#6028
started seeing verbose logging in admission webhook pods like:

`ts=2023-11-06T01:50:34.601774794Z caller=stdlib.go:105 caller=server.go:3215 msg="http: superfluous response.WriteHeader call from main.newSrv.func1 (main.go:173)"
`

Go doc says If WriteHeader is not called explicitly,
the first call to Write will trigger an implicit
WriteHeader(http.StatusOK).

This fix is to change the order of the method invocation
so log doesn't complain about this

Signed-off-by: Jayapriya Pai <slashpai9@gmail.com>
(cherry picked from commit 374f4b3)

cmd/admission-webhook/main.go

@@ -169,8 +169,8 @@ func newSrv(logger log.Logger, tlsConf *tls.Config) *srv {
 	mux.Handle("/metrics", promhttp.HandlerFor(r, promhttp.HandlerOpts{}))
 
 	mux.HandleFunc("/healthz", func(w http.ResponseWriter, r *http.Request) {
-		w.Write([]byte(`{"status":"up"}`))
 		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"status":"up"}`))
 	})
 
 	httpServer := http.Server{