Remove dependency on deprecated WebApiCompatShim

[johnthagen]

Closes #1214

This is a follow on of the work that has proceeded to eliminate remaining usages of deprecated ASP.NET 2.2 features from the code base.

• Upgrade backend to .NET 5 #1198
• Update backend dependencies #1215

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This change is 

[codecov-commenter]

Codecov Report

Merging #1220 (5e617f5) into master (cf05e0d) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1220   +/-   ##
=======================================
  Coverage   45.37%   45.37%           
=======================================
  Files         266      266           
  Lines        8110     8110           
  Branches      528      528           
=======================================
  Hits         3680     3680           
  Misses       4014     4014           
  Partials      416      416           

Flag	Coverage Δ
backend	58.43% <0.00%> (ø)
frontend	33.52% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
Backend/Controllers/UserController.cs	41.07% <0.00%> (ø)

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[imnasnainaec]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johnthagen)