Make Microsoft.AspNetCore.TestHost package compatible with .NET6 (ope…

…n-telemetry#1084)

Fixes NU1202	Package Microsoft.AspNetCore.TestHost 7.0.7 is not compatible with net6.0

Co-authored-by: Carter Socha <43380952+cartersocha@users.noreply.github.com>
Co-authored-by: Juliano Costa <julianocosta89@outlook.com>

src/cartservice/tests/cartservice.tests.csproj

@@ -6,7 +6,7 @@
 
   <ItemGroup>
     <PackageReference Include="Grpc.Net.Client" Version="2.53.0" />
-    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="7.0.7" />
+    <PackageReference Include="Microsoft.AspNetCore.TestHost" Version="6.0.21" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.6.2" />
     <PackageReference Include="xunit" Version="2.4.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5">