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

Updates #cleanup

Go 1.22.0 introduced the ability to use more expressive routing patterns
that include HTTP method when constructing ServeMux entries.
Applications that attempted to use these patterns in combination with
the old `tsweb.Debugger` would experience a panic as Go would not permit
the use of matching rules with mixed level of specificity.

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("/debug/", ret)
+	mux.Handle("GET /debug/", ret)
 
 	ret.KVFunc("Uptime", func() any { return varz.Uptime() })
 	ret.KV("Version", version.Long())