Flip extensionsAsCoreFeatures flag
Created by: philipp-spiess
Part of #40460
This PR removes the extensionsAsCoreFeatures
feature flag and changes its call sites to make it always true. After this is merged, migrated extensions are enabled by default.
Note: Merging this PR will flip the flag for our continuously deployed instances unless otherwise configured. I'll merge this later this week after we have done some testing with the flags turned on.