Update to .NET 7 & some project configuration improvements.

[Fabi]

Move shared project configurations to default.props. This allows easier updating of project files for future versions/changes.

Updating to .NET 7 alone improves memory allocations & usage a bit (up to 20%).

[jackpoz]

Not sure why appveyor is set up to build on Ubuntu too, but https://www.appveyor.com/docs/linux-images-software/ doesn't support .NET 7 SDK yet

[Fabi]

Not sure why appveyor is set up to build on Ubuntu too, but https://www.appveyor.com/docs/linux-images-software/ doesn't support .NET 7 SDK yet

Might be worth either removing ubuntu or switch to github actions on latest ubuntu lts (with installing .net7 if its not available yet) or just install .net 7 on appveyor ubuntu. That's not uncommon to do when those CI people are slow :D

[mdX7]

github actions might actually be an idea