NSubstitute 6.0.0 roadmap proposal

[Romfos]

I propose to implement changes in following areas for the next major release:

Infrastructure:

• Update tfm list: net8.0 + netstandard2.0
• Drop EOL .NET 6 and .NET 7 from test matrix
• Update dependencies
• Migrate documentation to docfx and remove ruby dependency (Migrate documentation to docfx #850)

.NET 9 support: (#848)

• Migrate to .NET 9 sdk and C# 13
• Add .NET 9 to test matrix

Build:

• Migrate from build "project" to .NET cli + GitHub actions
  • Release workflow Release build workflow #797
  • Migrate to dotnet pack Move package creating from build.fsproj to github actions #838

Code cleanup:

• Remove already obsoleted methods\classes that were obsolete for years
• Mark as obsolete some old classes\methods for "compatibility" with C# pre 7.0 and older versions

Public api changes:

• Remove #nullable disable annotations from code base and enable NRE for public api
• Make sealed all classes that could be sealed
• Make internal all classes that could be internal

cc @dtchepak @alexandrnikitin @tpodolak

[appel1]

Please continue to support .NET Framework as well. There's no EOL for .NET Framework >= 4.7 yet and being able to run our tests for all targets is a must for us.

https://learn.microsoft.com/en-us/lifecycle/products/microsoft-net-framework

[304NotModified]

Related #551