Update from .NET 6 to .NET 8

[par456]

.NET 6 hit it's end of life last month so it's time once again to move up our .NET version to .NET 8

This will have to be done very carefully, as we have a lot of packages and such that apsim relies on that will also need updating to the .net 8 versions. Once completed, we should be able to undo #9501 so that the warnings become errors again for the future.

Hopefully this process is a lot less painful than the 3.1 to 6.0 update from a few years ago.

In the meantime until we do this update, track any versioning problems with packages or visual studio here so that we can know to test them again once updated.