Invert booleans in has_create_events? method

Details !35154 (comment 367545341)

Agree, the default case is that create events are implemented. Only Upload/CI::JobArtifact/LfsObject are special cases which have this need.

We could e.g. refactor this method to better convey intent, e.g. invert the boolean, and call it .use_backfill_to_sync_new_files_hack?.

And BaseRegistry can implement it to return false so we don't need to implement it on any new SSF registry.