Upgrade dependencies for netstandard2.0 targeting

[akunzai]

• ASP.NET Core 2.1 becomes ASP.NET Core 2.3 dotnet/announcements#331

[codecov]

Codecov Report

Attention: Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.

Project coverage is 68.38%. Comparing base (2e24220) to head (655c527).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/GSS.Authorization.OAuth/AuthorizerBase.cs	66.66%	1 Missing and 1 partial ⚠️
...thorization.OAuth2.HttpClient/OAuth2HttpHandler.cs	0.00%	0 Missing and 1 partial ⚠️
...on.OAuth2/Authorizers/AccessTokenAuthorizerBase.cs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
- Coverage   68.97%   68.38%   -0.60%     
==========================================
  Files          24       24              
  Lines         606      601       -5     
  Branches       96       96              
==========================================
- Hits          418      411       -7     
  Misses        140      140              
- Partials       48       50       +2     

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