Change integration test to specify .net 6.0 as target framework

Author: tamasvajk

csharp/ql/integration-tests/all-platforms/cshtml_standalone_net6/cshtml.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net8.0</TargetFramework>
+    <TargetFramework>net6.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>