[Fleet] Disable FQDN selector by default (#152592)

## Summary

Hide the FQDN selector unless experimental feature is enabled. This
selector was added as part of
#149059

### Before:
<img width="1073" alt="Screenshot 2023-03-02 at 16 15 39"
src="https://user-images.githubusercontent.com/3315046/222487164-75024608-b0f1-419d-bbb0-299bad55cae1.png">

### After:
<img width="1112" alt="Screenshot 2023-03-02 at 16 14 52"
src="https://user-images.githubusercontent.com/3315046/222487206-c58f76eb-6977-4d3a-908d-3b72b0f901fa.png">

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

x-pack/plugins/fleet/common/experimental_features.ts

@@ -19,7 +19,7 @@ export const allowedExperimentalValues = Object.freeze({
   experimentalDataStreamSettings: false,
   displayAgentMetrics: true,
   showIntegrationsSubcategories: false,
-  agentFqdnMode: true,
+  agentFqdnMode: false,
   showExperimentalShipperOptions: false,
   fleetServerStandalone: false,
 });