Unified Build control update

[mmitche]

Description

This issue is dependent on dotnet/source-build#2974 and is a placeholder for .NET 8 required work in product repositories.

Instructions for implementation will be updated once dotnet/source-build#2974 but will likely be a mechanical shift of properties in this repo. In a general sense, this is scaffolding work for expanding the set of source-built platforms beyond Linux.

[mmitche]

This is .NET 8 work

[mmitche]

Blocked on move to net9 arcade

[rainersigwald]

@mmitche what's the timeline y'all expect for us to move to Arcade 9? Sooner than "after the last 8.0.X00 release"?

[mmitche]

I think that's right? We're not trying to drive a specific schedule, other than before .NET 9 ships. When would you normally move?

[rainersigwald]

Historically we try to move as soon as possible after our main branch will no longer ship in a previous SDK major.

[mmitche]

Resolved