Upgrade Assistant isn't compatible with newer XML solution formats

[Lamparter]

• Include the log file upgrade-assistant.clef that is produced in the working directory
  Upgrade assistant process does not even begin.

Describe the bug

Upgrade Assistant appears to be incompatible with newer XML solution formats introduced in the latest version of Visual Studio, the "SLNX" file format.

To Reproduce

• Create a new project
• Save as SLNX file
• Upgrade Assistant won't detect any projects

Further technical details

• Windows version and bitness info (32-bit or 64-bit)
  10.0.26100.3194 x64
• Include the output of dotnet --list-sdks

  8.0.309 [C:\Program Files\dotnet\sdk]
  9.0.103 [C:\Program Files\dotnet\sdk]
  9.0.200 [C:\Program Files\dotnet\sdk]
  

• Include the output of upgrade-assistant --version
0.5.829.41291 (0.5.829)