feat: add support for .NET6

[alsami]

• Add TFM NET6.0 to src- and test-project
• Update Sandbox sample for NET5.0 to use NET6.0 bc it can be used the same way
• Update build-files
• Update a few dependencies

Closes #41

[codecov]

Codecov Report

Merging #42 (61a1ed7) into develop (65c358e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop      #42   +/-   ##
========================================
  Coverage    83.11%   83.11%           
========================================
  Files            6        6           
  Lines           77       77           
  Branches         9        9           
========================================
  Hits            64       64           
  Misses          11       11           
  Partials         2        2           

Impacted Files	Coverage Δ
...t/AutofacMultitenantServiceCollectionExtensions.cs	100.00% <ø> (ø)
...tenant/AutofacMultitenantServiceProviderFactory.cs	88.88% <ø> (ø)
...nant/AutofacMultitenantWebHostBuilderExtensions.cs	100.00% <ø> (ø)
...ultitenant/MultitenantRequestServicesMiddleware.cs	100.00% <100.00%> (ø)

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[tillig]

LGTM - thanks!

[tillig]

I don't have time to cut a release today. It might be tomorrow or the next day. If someone else has time, awesome, otherwise it'll have to wait.

[alistairjevans]

I'm putting a release together now.

[alistairjevans]

https://github.com/autofac/Autofac.AspNetCore.Multitenant/releases/tag/v5.1.0