Update README.md

Author: mcatanzariti

README.md

@@ -7,7 +7,6 @@ High-performance [CBOR](https://cbor.io/) serialization framework for .Net (C#)
 
 ## Supported .NET versions
 * .NET Standard 2.0
-* .NET Core 3.1
 * .NET 6.0
 * .NET 7.0