Flip default values for feature flags before 4.0 release

Created by: philipp-spiess

• enableLegacyExtensions to false
• extensionsAsCoreFeatures to true

We also need to find all places where enableLegacyExtensions is initialized by default if ExperimentalExtensions is not set and change the logic of how nil is interpreted (cf. #40469)

This issue might be CCed in pull requests that add code that needs change once we work on it.

/cc @muratsu @jjinnii @ryankscott