Update build and packaging

[tillig]

While the project was already building using .NET 6, it wasn't doing so in a cross-platform manner (i.e., it couldn't build on Mac) and we weren't taking advantage of some of the newer language features. This is a housekeeping update for that.

• Updated analyzers and rulesets.
• Enabled nullable annotations and resolved warnings.
• Enabled implicit usings and cleaned up directives.
• Switched to file-scoped namespaces.
• Applied minor suggested refactorings like using pattern matching and simplifying null checks.
• Symbols will be published as .snupkg.
• README reformatted and included in .nupkg.

[codecov]

Codecov Report

Merging #70 (a7e365d) into develop (eb579aa) will not change coverage.
The diff coverage is 0.00%.

❗ Current head a7e365d differs from pull request most recent head 386fe5d. Consider uploading reports for the commit 386fe5d to get more accurate results

@@           Coverage Diff           @@
##           develop     #70   +/-   ##
=======================================
  Coverage     0.00%   0.00%           
=======================================
  Files           23      23           
  Lines          677     685    +8     
  Branches       137     146    +9     
=======================================
- Misses         677     685    +8     

Impacted Files	Coverage Δ
...tion.WebApi/AuthenticationFilterOverrideWrapper.cs	0.00% <0.00%> (ø)
....Integration.WebApi/AuthenticationFilterWrapper.cs	0.00% <0.00%> (ø)
...ation.WebApi/AuthorizationFilterOverrideWrapper.cs	0.00% <0.00%> (ø)
...c.Integration.WebApi/AuthorizationFilterWrapper.cs	0.00% <0.00%> (ø)
...c.Integration.WebApi/AutofacActionFilterAdapter.cs	0.00% <0.00%> (ø)
....WebApi/AutofacControllerConfigurationAttribute.cs	0.00% <0.00%> (ø)
...utofac.Integration.WebApi/AutofacOverrideFilter.cs	0.00% <0.00%> (ø)
...egration.WebApi/AutofacWebApiDependencyResolver.cs	0.00% <0.00%> (ø)
...Integration.WebApi/AutofacWebApiDependencyScope.cs	0.00% <0.00%> (ø)
....Integration.WebApi/AutofacWebApiFilterProvider.cs	0.00% <0.00%> (ø)
... and 26 more

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[alistairjevans]

☂