Target .NET 10

[RussKie]

This change does the following:

• Introduces an MSBuild property, TargetDotNet10, which is set to true only in CI by default, so that when building on a machine without .NET 10 you don't get a build error. You can manually install .NET 10 in a developer environment and set that environment variable to build and debug locally.
• Updates MSBuild package to 17.12.6 and uses the net9.0 reference assemblies
• Adds two NuGet feeds in order to get .NET 10 packages
• Updates the build YAML to install daily builds of .NET 10 in CI. At build time, the runtimeconfig.json still just points to any .NET 10 runtime so we don't have to worry about always installing the same version.
• Added logic to just fail on version of MSBuild older than 17.0 since net461 was dropped.
• Updated tests that get solution folders under .NET 10 since those APIs in MSBuild changed and only list projects now for some reason.
• Update SlnGen version to 12.0

[RussKie]

@jeffkl, I could use some help. I can build it locally but somehow it stumbles on the CI.

[jeffkl]

Okay I got everything working except a few tests which are throwing strange errors, almost like a behavioral difference in .NET 10, can you debug them @RussKie ?

[jeffkl]

@RussKie never mind, I think I figured it out, new MSBuild has new APIs for parsing solutions 😢 I'll see if I can fix it...

[RussKie]

We need to use a specific early version, otherwise we'll get a repeat of #456 - i.e., a consumer may have an earlier version of .NET 10 installed than a version which was used to build the slngen tool.

[RussKie]

ditto

[RussKie]

Thank you, @jeffkl!

[RussKie]

I tested it locally against a dotnet/extensions's .NET 10 branch, and all worked as expected. This is good to merge.

[mruxmohan4]

Thank you for your contribution!