im: IdentityModel.Tests remove net6.0 target and add net9.0

Author: damianh

identity-model/test/IdentityModel.Tests/IdentityModel.Tests.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net6.0;net8.0</TargetFrameworks>
+    <TargetFrameworks>net8.0;net9.0</TargetFrameworks>
     <RootNamespace>Duende.IdentityModel</RootNamespace>
     <Nullable>disable</Nullable>
   </PropertyGroup>