The list on the page you reference is the versions of SSRS that are supported to upgrade FROM. So, basically any version of SSRS from 2008 onwards is supported to upgrade to SQL 2017.
https://learn.microsoft.com/en-us/sql/reporting-services/install-windows/migrate-a-reporting-services-installation-native-mode?view=sql-server-ver15
...one of the following supported versions of a Reporting Services native
mode deployment to a new SQL Server Reporting Services instance:
SQL Server 2016 (13.x)
SQL Server 2014 (12.x)
SQL Server 2012 (11.x)
SQL Server 2008 R2
SQL Server 2008
Yes, the migration assistant is useful, as it mentions it will check for non-standard configurations of the source SSRS installation to make sure that nothing weird will happen.
By running Upgrade Advisor, you can find out about custom settings
that might not be supported in a new SQL Server installation.
My preference though is not to migrate/update SSRS but to do a fresh deployment from source control (upgrade the project to the target version) and go from there. This means that you will need to recreate the schedule for any reports, etc, etc. But this is a good time to go over your SSRS installation and cull the cruft that has built up. Look at the execution history, find out which reports are actually used! Make sure that they are sharing data sources and you don't have any wonky connections that are over privileged. It's more work but you are making your environment better by taking the time.
