Removed .NET 8.0 support

Author: timglewis

src/SharpGLTF.Core/SharpGLTF.Core.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>    
-    <TargetFrameworks>netstandard2.0;netstandard2.1;net6.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;netstandard2.1;net6.0</TargetFrameworks>
     <AssemblyName>SharpGLTF.Core</AssemblyName>
     <RootNamespace>SharpGLTF</RootNamespace>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>