Upgraded to Net 8.0

[jakenuts]

Not sure if you need this but I took the morning to upgrade the project to Net 8.0.

Along the way I

• Updated all the packages to latest
• Fixed some basic syntax issues (like disposing of classes that had using var mything already)
• Fixed namespace handling for file scoped namespaces
• Fixed some source generation issues I ran into with the latest packages.
• And probably unforgivably I ran Resharper's "Cleanup Code" across the solution which makes for a ton of changes but also uses newer/more succinct syntax where possible.
• Got green tests for all the OpenAI stuff but I don't have an Azure key so can't confirm 100% green.

Anyhoo, feel free to say "nope" and close this.

Thanks!

James