Update to Autofac 8; add .NET 8 support; drop .NET 3.1

[tillig]

PR is waiting on this Autofac.Multitenant PR to get some updates on the nullable annotations.

---

Fixes #50

• Update semver from 7.x => 8.0.0 to account for the breaking Autofac change coming.
• Add support for .NET 8.
• Drop support for .NET 3.1.
• Update dependencies:
  • Autofac.Extensions.DependencyInjection => 9.0.0
  • Autofac.Multitenant => 8.0.2
• Change the sandbox app to not specify a range of versions - just .NET 6 right now. If we have one sandbox app per .NET version, it will be cleaner to add/remove versions - we won't have to rename things. (This caused a lot of churn in this PR.)
• Fix analyzer warnings based on the addition of .NET 8.
• Switch from Moq to NSubstitute.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3f32a8f) to head (5e8dc65).

Additional details and impacted files

@@             Coverage Diff              @@
##           develop       #51      +/-   ##
============================================
+ Coverage    97.14%   100.00%   +2.85%     
============================================
  Files            6         6              
  Lines           70        67       -3     
  Branches         7         4       -3     
============================================
- Hits            68        67       -1     
+ Misses           1         0       -1     
+ Partials         1         0       -1     

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[alistairjevans]

Looks fine, full disclosure havent looked in great detail at all the sandbox updates, but look ok on the surface.