Use NullabilityInfoContext to determine if member is nullable

[patrikwlund]

Add-on to #3043 (should be no conflict)

Very similar to my previous PR: #3041

In his PR, @VladimirTyrin surgically fixes a specific problem for all platforms. I suggest we use the new and fancy NET6+ toys when available, to avoid custom nullable logic completely in the future. I tested it with the same added test as in @VladimirTyrin's original PR.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.18%. Comparing base (f91f560) to head (ec62df7).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
...SwaggerGen/SchemaGenerator/MemberInfoExtensions.cs	72.72%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3046      +/-   ##
==========================================
- Coverage   90.49%   90.18%   -0.31%     
==========================================
  Files          74       74              
  Lines        2987     2964      -23     
  Branches      471      463       -8     
==========================================
- Hits         2703     2673      -30     
- Misses        284      291       +7     

Flag	Coverage Δ
Linux	90.18% <72.72%> (-0.31%)	⬇️
Windows	90.18% <72.72%> (-0.31%)	⬇️
macOS	90.18% <72.72%> (-0.31%)	⬇️

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[patrikwlund]

Ended up being a conflict anyway, so I will rebase shortly.

[martincostello]

That's not necessary. I've fixed the conflict and enabled squash merge.