Drop .NET 6

[martincostello]

Drop support for .NET 6.

Resolves #3087.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.56%. Comparing base (730b315) to head (d549578).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3183   +/-   ##
=======================================
  Coverage   83.56%   83.56%           
=======================================
  Files          77       77           
  Lines        3224     3224           
  Branches      559      559           
=======================================
  Hits         2694     2694           
  Misses        530      530           

Flag	Coverage Δ
Linux	83.56% <ø> (ø)
Windows	83.56% <ø> (ø)
macOS	83.56% <ø> (ø)

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[Saibamen]

Please remove all #if NET6_0_OR_GREATER and #if NET7_0_OR_GREATER.

And if these IF statements have ELSE, update check to #if NET8_0_OR_GREATER. <----- EDIT: Umm, no, this is Swashbuckle.AspNetCore, not NSwag :D - we are not supporting .NET Framework

[martincostello]

No, that's unnecessary code churn that makes no difference just changing the version numbers.

The elses may be removable in some cases, but only for projects that don't target netstandard2.0.