Remove DiagnosticSource ref for NET 6+ (#2033)

Remove System.Diagnostics.DiagnosticSource reference from Extensions for NET 6.0 and later.

Author: lahma

src/Polly.Extensions/Polly.Extensions.csproj

@@ -41,7 +41,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.Extensions.Options" />
     <PackageReference Include="Microsoft.Extensions.Logging.Abstractions" />
-    <PackageReference Include="System.Diagnostics.DiagnosticSource" />
+    <PackageReference Include="System.Diagnostics.DiagnosticSource" Condition="!$([MSBuild]::IsTargetFrameworkCompatible($(TargetFramework), 'net6.0'))" />
     <Reference Include="System.Net.Http" Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'" />
   </ItemGroup>
 </Project>