Allow the use of .NET Core 10

[freakboy3742]

Allows the Winforms backend to use .NET Core 10, instead of .NET Framework 4.x.

This is required to add native Windows on ARM support - Windows 10 and 11 both provide .NET Framework 4.x out of the box, but on ARM machines, it's an x86-64 build.

Fixes #2782.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Two issues revealed by CI testing:

1. There's a chicken and egg problem: We can't run testbed on Toga until Briefcase supports ARM; we can't run CI on Briefcase until Toga supports ARM.
2. There are a handful of APIs that are actually different on .NET Core. This appears to be mostly an issue with MenuItem and StatusIcon objects. I'm not yet sure if the best solution is to hard switch to .NET core, or if there's a compatibility path that will allow both.