Use file-scoped namespaces

[MitchRazga]

C# 10 added file-scoped namespaces, which allow you to declare that every type within a file belongs to that namespace.
This effectively removes 1 level of nesting/indentation from every file.
Read more here
Since this repo targets multiple TFMs we need to set the LangVersion explicitly so it does not choose the default eg. C# 7.3.

Proposal
Set <LangVersion>latest</LangVersion> per .csproj or in a Directory.Build.props

Additional context
File-scoped namespaces is now standard in many .NET repos.