Runtime and dependencies upgrade

[paoloambrosio]

Upgrade .NET runtime and all dependencies to the latest stable.

Notes:

• Rx.NET does not have a .NET 6 build (see https://github.com/dotnet/reactive/issues/1725), though we don't necessarily require it.

[paoloambrosio]

Tests seemed to take way more after the upgrade, even when they succeed. For reference, this is on the current main branch:

After upgrading (ignore the errors, see #42):

JetBrains dotTrace shows that AutoBogus is the bottleneck. I've opened an issue to document it: https://github.com/nickdodd79/AutoBogus/issues/86. Issue https://github.com/nickdodd79/AutoBogus/issues/12 might be loosely related.

Good news is that the generation can now be accessed concurrently. Making the generated fixture static made unit tests as quick as we'd expect.