Remove net6.0 target framework.

Author: wo80

CSparse/CSparse.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>netstandard2.0;net6.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;net8.0</TargetFrameworks>
     <GeneratePackageOnBuild>False</GeneratePackageOnBuild>
     <GenerateDocumentationFile>True</GenerateDocumentationFile>
     <PackageId>CSparse</PackageId>