Update SharpGLTF.Cesium.Tests.csproj

tests/SharpGLTF.Cesium.Tests/SharpGLTF.Cesium.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net471;net8.0-windows</TargetFrameworks>
+    <TargetFrameworks>net471;net6.0-windows;net8.0-windows</TargetFrameworks>
     <IsPackable>false</IsPackable>
     <RootNamespace>SharpGLTF</RootNamespace>
     <LangVersion>latest</LangVersion>