Fixing legacy bug for Plot Over Line

Plot Over Line backen changed recently, and its proxy API changed. State files (.pvsm) produced with master between 5.9 and 5.10 wrongly downgraded the proxy to be loaded to the legacy one. This is because current builds are tagged as 5.9, even if it already has changes that are targetted for 5.10. Since the proxy API is different, we can discriminate "true" 5.9 instances of .pvsm files from the ones built with master after the backend was changed. In particular, Point1 and Point2 properties are new properties that didn't use to be there.

Fixes #20778 (closed)