Smart PropertyGrid and Smart FieldPackEditor for .Net 8

.Net 8 assemblies have just been added to the nuget packages.

• SPG for .Net 6+ is now version 4.2
• SFPE for .Net 6+ is now version 2.2.1

On this occasion, I had to update SPG to fix a .Net 8 regression in the KeysConverter class. I've raised this issue with the Microsoft development team and they should fix it for .Net 8 and .Net 9 preview.