Upgrade to dotnet9 and tw v4

[tesar-tech]

closes #9

• Just the ordinal update of nuget and framework for .net9.
• For the tw it is just about the migrating the tailwind.config.js to app.css...
• the ubuntu-latest doesn't have the dotnet preinstalled.

todos:

• check the readme
• add .net 9 assets functionality
• probably change the index gradient as it is too bright... (heh)