Bump NuGet packages

[martincostello]

• Update Microsoft.OpenApi to v1.6.23.
• Update ASP.NET Core 2.1 packages to ASP.NET Core 2.3.

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.54%. Comparing base (4252bef) to head (c10049e).
Report is 1 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3319      +/-   ##
==========================================
- Coverage   84.84%   83.54%   -1.31%     
==========================================
  Files          77       77              
  Lines        3221     3221              
  Branches      556      556              
==========================================
- Hits         2733     2691      -42     
- Misses        488      530      +42     

Flag	Coverage Δ
Linux	83.54% <ø> (+17.23%)	⬆️
Windows	83.54% <ø> (+17.23%)	⬆️
macOS	83.54% <ø> (ø)

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