Update to Autofac 8.

[tillig]

Update for Autofac 8 compatibility. Includes:

• Updates to build metadata (.editorconfig, VS Code settings, analyzer rulesets) to match latest Autofac where possible.
• Update dependencies:
  • Autofac 6.4.0 => 8.0.0
  • System.ComponentModel.Composition 4.5.0 => 8.0.0
• Add support for net8.0, net7.0, net6.0 to match both Autofac 8 and System.ComponentModel.Composition 8.
• Fix nullability warnings that popped up with new dependencies, frameworks.

Fixes #28.

[codecov]

Codecov Report

Attention: Patch coverage is 82.35294% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 72.56%. Comparing base (05de15e) to head (5bac5d0).

Files	Patch %	Lines
.../Autofac.Integration.Mef/RegistrationExtensions.cs	85.71%	0 Missing and 2 partials ⚠️
...tofac.Integration.Mef/Util/ReflectionExtensions.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #29      +/-   ##
===========================================
+ Coverage    71.18%   72.56%   +1.38%     
===========================================
  Files            8        7       -1     
  Lines          347      339       -8     
  Branches        67       68       +1     
===========================================
- Hits           247      246       -1     
+ Misses          69       61       -8     
- Partials        31       32       +1     

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[alistairjevans]

Just one comment, a slight inconsistency in the use of ! vs doing a null check+throw. No big deal though if there's a reason.