remove .net core 2.1 from testing

Author: Shane32

tests/Transports.AspNetCore.Tests/Transports.AspNetCore.Tests.csproj

@@ -2,7 +2,7 @@
   <Import Project="../../Tests.props" />
 
   <PropertyGroup Condition="'$([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::Windows)))' != 'true'">
-    <TargetFrameworks>netcoreapp2.1;net5.0;net6.0;net7.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>net5.0;net6.0;net7.0;net8.0</TargetFrameworks>
     <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>
   </PropertyGroup>
   <PropertyGroup Condition="'$([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::Windows)))' == 'true'">