How to migrate SQL Server 2014 to SQL Server 2019 on Linux using AG
You cannot add a Linux system to a Windows cluster, so you won't be able to add any Linux systems into a SQL Server 2014 availability group. You also cannot do the migration with distributed availability groups because SQL Server 2014 did not have support for distributed availability groups.  From Distributed availability groups:


  Because the distributed availability groups feature did not exist in SQL Server 2012 or 2014, availability groups that were created with these versions cannot participate in distributed availability groups.


In order to be able to do a migration using availability groups, you would have to get the SQL Server 2014 systems upgraded to at least SQL Server 2017 where you can create an availability group with a cluster type of "none," which will allow you to add a replica to a Linux SQL Server.  See Always On Availability Groups on Linux -> Interoperability with Windows-based availability groups and replicas.

Always On Availability Groups on Linux states that you can use Pacemaker, or no cluster for Linux availability groups.