Update .NET version to 9.x in publish_aspnetcore.yml

The .NET version specified in the publish_aspnetcore.yml file has been updated from 8.x to 9.x. This change updates the environment variable NET_VERSION to ensure the project targets .NET version 9.x.

.github/workflows/publish_aspnetcore.yml

@@ -7,7 +7,7 @@ on:
   workflow_dispatch:
 
 env:
-  NET_VERSION: '8.x'
+  NET_VERSION: '9.x'
   PROJECT_NAME: src/TinyHelpers.AspNetCore 
   PROJECT_FILE: TinyHelpers.AspNetCore.csproj
   TAG_NAME: aspnetcore