Extension deprecation: Change enableLegacyExtensions flag to default to true

Created by: philipp-spiess

This PR reverts #39661

Unfortunately fixing the typo revealed a mistake in the JSON struct parsing logic. Since ExperimentalFeatures is defined as a Go struct, its boolean members will always default to false. It appears that the JSON parsing logic is leaving the default value when no property is defined in the config file and thus enableLegacyExtensions is set to false potentially disabling the extensions (even though this should only be the behavior after 4.0)

Test plan