.NET 8 support

[anatawa12]

ubuntu-latest runner image is going to use ubuntu-24.04 instead of ubuntu-22.04 since this week.

However, ubuntu-24.04 does not include .NET 6 so we cannot run this action without manually installing .NET 6. (see vrchat-community/template-package#22).
In ubuntu-24.04 runner image, .NET 8 are included so I want vrchat-community/package-list-action to have .NET 8 support.

Please note that we cannot remove .NET 6 support from the HEAD branch of vrchat-community/package-list-action because we can use vrchat-community/package-list-action in self-hosted github actions runners without .NET 6 now.