This one is a tough one 
I would recommend at least 7.
Version 6 reaches EOL in two months.
Version 7 has also been out for a good while Aug 2023. The really complicated part is going to be that you can’t just jump. you’ll have to do it sequentially. this post i think would help the most in that: Dump and Restore between version 3.6 and 5.0 and then after that 5->6->7 is possible.