http: improve UI not enabled response message

Response now clearly indicates:
- the UI is disabled
- how to enable the UI

.changelog/11820.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+http: when a user attempts to access the UI but can't because it's disabled, explain this and how to fix it
+```

agent/http.go

@@ -593,7 +593,7 @@ func (s *HTTPHandlers) Index(resp http.ResponseWriter, req *http.Request) {
 	// Give them something helpful if there's no UI so they at least know
 	// what this server is.
 	if !s.IsUIEnabled() {
-		fmt.Fprint(resp, "Consul Agent")
+		fmt.Fprint(resp, "Consul Agent: UI disabled. To enable, set ui_config.enabled=true in the agent configuration and restart.")
 		return
 	}