Can’t build due to 7.0 TargetFrameworkVersion

### Describe the bug

I am trying to build WinUI 3 from source. When I do so, I get multiple errors related to the fact that the `$(TargetPlatformVersion)` MSBuild property is set to its default of 7.0 (by the vcxproj targets). This should be set in `eng\Versions.props`, but for some reason it isn’t. See attached binlog.

### Why is this important?

I want to contribute to WinUI 3, and to do that I need to be able to build it.

### Steps to reproduce the bug

1. Check out commit d0c6705ec46dbddb853cdfdb2eb3b42849cf304e.
2. Run `build.cmd /i arm64chk`.
3. Observe the error in the output.

### Actual behavior

Build fails.

### Expected behavior

Build should succeed

### Screenshots

N/A

### NuGet package version

N/A, source build issue

### Windows version

Windows 11 (25H2): Build 26200

### Additional context

[Here is a zipped binlog.](https://github.com/user-attachments/files/32025029/binlog.zip)

Start at eng\Versions.props and inspect how the vcxproj targets set $(TargetPlatformVersion). Reproduce the failure at commit d0c6705ec46dbddb853cdfdb2eb3b42849cf304e with build.cmd /i arm64chk, using the attached binlog if needed. Done means the source build succeeds without the 7.0 TargetPlatformVersion errors.

Written by the indexing model from the issue text.