Update contributing.md to say you need the .NET 8 SDK rather than .NET 6 (#130)

Author: Numpsy

CONTRIBUTING.md

@@ -153,8 +153,8 @@ from the main (upstream) repository:
 
 ## <a name="build"></a> Build process
 
- * You need [.NET 6.0 SDK](https://dotnet.microsoft.com/download/dotnet/6.0)
- * Run `dotnet tool restore` to restore the .NET 6 local tools defined at .config/dotnet-tools.json
+ * You need [.NET 8.0 SDK](https://dotnet.microsoft.com/download/dotnet/8.0)
+ * Run `dotnet tool restore` to restore the .NET local tools defined at .config/dotnet-tools.json
  * To build the project run `dotnet run` (this will run the `build.fsproj` project that contains the FAKE build pipeline.)
  * To run unit tests run `dotnet run Test`