.Net upgrade to 9.0

[elinohlsson]

Is your feature request related to a problem? Please describe.
Upgrade .NET version to 9.0

[ivara]

IMPORTANT

There is an issue in .NET 9 (9.0.101) that seems to prevent this upgrade
dotnet/runtime#110067

Pretty hefty diagnostics done by Nness in this comment
dotnet/runtime#110067 (comment)

[elinohlsson]

Update: The project has been upgraded directly from .NET 8 to .NET 10, skipping the short-term support release of .NET 9. See the v12.0.0 release notes for details on the .NET 10 update.

Since the upgrade has been completed, this issue is now being closed.