[Search] Enable semantic_text feature in es3 (elastic#197140)

## Summary

Semantic text feature was disabled by default due to ML node unable to
scale down. With the relevant
[PR](elastic/elasticsearch#114323) & subsequent
[fix](elastic/elasticsearch#115189) merged, ML
node now auto scales when there is no activity. Therefore enabling
semantic_text feature in es3.

### Testing instructions
* start serverless instance
* visit index management index details page -> mappings
* Click Add field 
* Confirm `semantic_text` is shown in the field type form

config/serverless.es.yml

@@ -120,4 +120,4 @@ xpack.searchInferenceEndpoints.ui.enabled: false
 xpack.search.notebooks.catalog.url: https://elastic-enterprise-search.s3.us-east-2.amazonaws.com/serverless/catalog.json
 
 # Semantic text UI
-xpack.index_management.dev.enableSemanticText: false
+xpack.index_management.dev.enableSemanticText: true