Add conditional TargetFrameworks for testing previewUpdate target framework

Author: JimBobSquarePants

tests/GeoJson/GeoJson.csproj

@@ -1,9 +1,21 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
-        <TargetFramework>net8.0</TargetFramework>
         <ImplicitUsings>enable</ImplicitUsings>
         <Nullable>enable</Nullable>
     </PropertyGroup>
 
+  <Choose>
+    <When Condition="$(SIXLABORS_TESTING_PREVIEW) == true">
+      <PropertyGroup>
+        <TargetFrameworks>net9.0;net8.0</TargetFrameworks>
+      </PropertyGroup>
+    </When>
+    <Otherwise>
+      <PropertyGroup>
+        <TargetFrameworks>net8.0</TargetFrameworks>
+      </PropertyGroup>
+    </Otherwise>
+  </Choose>
+
 </Project>