Describe the bug

Visual Studio Preview suddenly fails to build in all sorts of interesting ways on code that used to work perfectly well in .NET 6. Any project with a <TargetFrameworks> tag in its csproj rather than a <TargetFramework> tag is barfing everywhere.

To Reproduce

• Setup .NET 7 RC1 on the latest Visual Studio Preview, using the links found on this announcement.
• Create a new solution with two new projects, for .NET 7. Project 2 should have a dependency on project 1 and should use a type declared in project 1.
• Alter project 1's csproj to say <TargetFrameworks>net6.0;net7.0</TargetFrameworks> rather than net7.0`
• Try to rebuild the solution. Watch as everything goes wrong.

Expected behavior:
This was working fine in .NET 6. It should continue to work fine in .NET 7.

Observed behavior:
First, Visual Studio complains about TargetFrameworkMoniker and NuGetTargetFrameworkMoniker properties being missing from the project. Then you manually rebuild project 1 and it works, but when you try to rebuild project 2, it has trouble with the types declared in project 1. VS very helpfully suggests adding a reference to project 1, even though project 2 already has one. An option to do so even shows up in the relevant ALT-ENTER popup menu. Selecting this option does nothing.

How did such a massive regression in such a basic piece of functionality end up in an RC?!?