Upgrade from 3.4.5 to 6.0.1

Hello
Maybe this has been already discussed but I would like to ask because I cant find any detailed instructions.

I would like to upgrade my redmine installation 3.4.5 on SLES 12 to a new server with SLES 15 with redmine version 6.0.1 (Database MariaDB).

Can you help on detailed instructions and step by step guide ?
Do I need to perform an intermediate migration first ?
Is Redmine still supporting MariaDB ?