4.x Review Incubating/Experimental features

Review all features annotated `@Incubating` or `@Preview` and make sure it is still the case.
If the annotation is removed/modified, please update all related documentation, examples etc.

See https://github.com/helidon-io/helidon/blob/helidon-3.x/common/common/src/main/java/io/helidon/common/FeatureCatalog.java for correct 3.x setup.