Thank you Saragani, that worked and has enabled me to update my system to work with the new security enhancements on my Linux machine using .Net 6.
If anybody else is trying to do the same, in addition to changes to the code documented elsewhere to pass the password to the PLC, I also needed to add references to the following packages (all version 8):
System.Configuration.ConfigurationManager
System.IO.Ports
System.Security.Cryptography.ProtectedData