[csharp] Update to TargetFrameworkVersion to v4.8

spine-csharp/spine-csharp.csproj

@@ -10,7 +10,7 @@
     <AppDesignerFolder>Properties</AppDesignerFolder>
     <RootNamespace>Spine</RootNamespace>
     <AssemblyName>spine-csharp</AssemblyName>
-    <TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
+    <TargetFrameworkVersion>v4.8</TargetFrameworkVersion>
     <TargetFrameworkProfile>
     </TargetFrameworkProfile>
     <XnaFrameworkVersion>v4.0</XnaFrameworkVersion>