Initial conversion to Net 8.0

[zur003]

Working on #9503

This version compiles, but doesn't include package updates. Those are yet to come.

[zur003]

It looks like the docker image that Jenkins uses to build ApsimX will need to be update to use .Net 8.0 before this will work.

[par456]

Looking at the docker scripts for this, it's based on a 6.0 image, so it'll be a pain to add both 6 and 8 in at the same time, and if we change it to net 8, then all the other PRs won't run until this is merged. We could potentially install 8 as part of the build process, but that'll also blowout the build times while it's active like that.

Considering it's halfway through a friday, we might need to sit on this PR for another day where we do a planned switch across to 8 and get the conversion done in one jump and have time for all the followup testing it'll require.

Thanks for getting a start on this Eric, much appreciated.