tests: remove olde target frameworks

Author: LiorBanai

src/NetMQ.Tests/NetMQ.Tests.csproj

@@ -7,7 +7,7 @@
     <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
     <GenerateRuntimeConfigurationFiles>true</GenerateRuntimeConfigurationFiles>
     <IsTestProject>true</IsTestProject>
-    <TargetFrameworks>net6.0;netcoreapp3.1;net47</TargetFrameworks>
+    <TargetFrameworks>net9.0;net47</TargetFrameworks>
   </PropertyGroup>
 
   <PropertyGroup>