test: Update AspNetCoreMvcCoreFrameworkApplication to ASP.NET Core 2.3

[nr-ahemsath]

ASP.NET Core 2.3 is just a reshipping of ASP.NET Core 2.1. See Microsoft's announcement for details: dotnet/announcements#331 So these changes should basically be a no-op.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.78%. Comparing base (0e78da3) to head (58f351e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3525      +/-   ##
==========================================
- Coverage   81.79%   81.78%   -0.01%     
==========================================
  Files         508      508              
  Lines       34228    34228              
  Branches     4040     4040              
==========================================
- Hits        27996    27993       -3     
- Misses       5266     5268       +2     
- Partials      966      967       +1     

Flag	Coverage Δ
Agent	82.77% <ø> (-0.01%)	⬇️
Profiler	71.75% <ø> (ø)

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see 1 file with indirect coverage changes

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