Smart PropertyGrid and Smart FieldPackEditor for .Net 7

Since .Net 7 has been released last week, I have added .Net 7 assemblies to the nuget packages.

• SPG for .Net 6+ is now version 4.1
• SFPE for .Net 6+ is now version 2.1

Note that both products are newly labelled "for .Net 6+" to include current and upcoming .Net versions starting from 6.