Users shouldn't be able to change the number of retained generations

Currently, SegmentGCOptions implements a validation check on the number of retained generations that prevents callers to set it to a value smaller than two. Instead, some subsystems might benefit from having a number of retained generations smaller than two - see OAK-7157 as an example.

The check performed by SegmentGCOptions should be moved to SegmentNodeStoreService to prevent users from setting the number of retained generations to a an invalid value. Internal subsystems should be allowed to set the number of retained generations to a lower value, if needed.