Switch TFM from net8.0 to net9.0.

Closes #164.

Author: alexrp

Directory.Build.props

@@ -28,7 +28,7 @@
         <PolySharpIncludeRuntimeSupportedAttributes>true</PolySharpIncludeRuntimeSupportedAttributes>
         <PublishRelease>false</PublishRelease>
         <RepositoryUrl>https://github.com/vezel-dev/cathode.git</RepositoryUrl>
-        <TargetFramework>net8.0</TargetFramework>
+        <TargetFramework>net9.0</TargetFramework>
         <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     </PropertyGroup>