✨multi-target .net versions

[MafuJosh]

Support multiple versions of .net, including newer versions of .net framework.
in:

• main library
• demo apps
• unit testing library

Tasks:

• .NET Standard 2.0 (NET Framework 4.6.1 to 4.8.1)
• .NET Standard 3.0 (NET 3.0 to 7.0)
• .NET 6
• .NET 7
• .NET 8
• .NET Framework 4.6
• Determine what other frameworks to support, if any.
  • net35
  • net40
  • net403
  • net45
  • net451
  • net452
• Sync changes to code generator