Property Version

Used by DefaultSegmentInfosReader to upgrade a 3.0 segment to record its version is "3.0". this method can be removed when we're not required to support 3x indexes anymore, e.g. in 5.0.

NOTE: this method is used for internal purposes only - you should not modify the version of a SegmentInfo, or it may result in unexpected exceptions thrown when you attempt to open the index.

@lucene.internal