Enforce using .NET SDK >= 8

[paulloz]

As discussed on 🚀

Officially enforce that we are building the .NET projects with SDK 8.

• I believe we are already building with this on CI since we're running ubuntu-20.04.
• The build container already uses this SDK too.
• The required SDK version on the Compiling with .NET documentation page will need to be edited to reflect this change.
• This should not impact anything since we do not change any TFM, nor any C# version used.
• This will however close .NET SDK 6.0 can't be used anymore to build C# solutions in a project #89386 by not allowing to build with SDK 6 any more.

Since the lookup for global.json starts in the current working directory (and not in the project directory), I had to modify our Python script to make sure we run our MSBuild commands from the modules/mono/ folder.

[raulsntos]

Thanks, looks great. I imagine most contributors have upgraded to 8.0 by now.

[akien-mga]

Thanks!