BuildTools being compiled with C# 10 instead of 7.3

[devhawk]

BuildTools targets .NET Framework 4.72 and .NET Standard 2.0. The latest supported version of C# that targets these .NET frameworks is 7.3. However, this repo specifies C# 10 in the src/Directory.Build.props file.

Build Tools is currently using a few C# 8 and above features

• local functions
• switch expression
• nullable reference types
• 'is not null' patterns
• target-typed object creation
• record struct
• declaring a type inside an interface

[devhawk]

See https://github.com/Sergio0694/PolySharp