Upgrade to Zorin 16 from 15.3

[MaximX85]

Artyom addresses the upgrade path here:

Zorin OS 16 and beyond

We had originally planned to develop an upgrade path from Zorin OS 12 to 15, however there wasn’t enough time during the last development cycle to finish and test this feature. Zorin OS 15 was a rather ambitious release with a lot of new improvements which caused us many delays. Adding this would have delayed it even further, so we decided to not spread ourselves too thin. However, we’re planning to enable upgrades between Zorin OS 15 and 16, as this feature is now one of the most high-priority…