[TestApp.AspNetCore] Replace .NET 6 target with .NET 8 (open-telemetry#2184)

Author: joegoldman2

test/TestApp.AspNetCore/TestApp.AspNetCore.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
-    <TargetFrameworks>$(SupportedNetTargets)</TargetFrameworks>
+    <TargetFrameworks>$(SupportedNetTargetsWithoutNet6)</TargetFrameworks>
   </PropertyGroup>
 
   <ItemGroup>