Revert "tsweb: update ServeMux matching to 1.22.0 syntax (#11087)" (#11089)

This reverts commit 291f91d164.

Updates #cleanup

This PR needs additional changes to the registration of child handlers under /debug

Signed-off-by: Patrick O'Doherty <patrick@tailscale.com>

tsweb/debug.go

@@ -46,7 +46,7 @@ func Debugger(mux *http.ServeMux) *DebugHandler {
 	ret := &DebugHandler{
 		mux: mux,
 	}
-	mux.Handle("GET /debug/", ret)
+	mux.Handle("/debug/", ret)
 
 	ret.KVFunc("Uptime", func() any { return varz.Uptime() })
 	ret.KV("Version", version.Long())