Update to .NET Core 5 with EF Core 6 Preview 3

[bart-degreed]

This PR shows that using EF Core 6 Preview 3, all of our existing tests succeed with the workaround for dotnet/efcore#21026 removed.

This PR is not meant to be merged.

[codecov]

Codecov Report

Merging #978 (bf241bd) into master (20308b0) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head bf241bd differs from pull request most recent head c6c2b74. Consider uploading reports for the commit c6c2b74 to get more accurate results

@@            Coverage Diff             @@
##           master     #978      +/-   ##
==========================================
+ Coverage   91.26%   91.29%   +0.03%     
==========================================
  Files         281      281              
  Lines        7429     7425       -4     
==========================================
- Hits         6780     6779       -1     
+ Misses        649      646       -3     

Impacted Files	Coverage Δ
...tCoreExample/Definitions/ArticleHooksDefinition.cs	100.00% <100.00%> (ø)
...cOperations/Processors/SetRelationshipProcessor.cs	100.00% <100.00%> (ø)
...NetCore/Configuration/InverseNavigationResolver.cs	100.00% <100.00%> (ø)
...NetCore/Configuration/JsonApiApplicationBuilder.cs	100.00% <100.00%> (ø)
.../JsonApiDotNetCore/Configuration/JsonApiOptions.cs	100.00% <100.00%> (ø)
...otNetCore/Configuration/JsonApiValidationFilter.cs	100.00% <100.00%> (ø)
src/JsonApiDotNetCore/Configuration/PageNumber.cs	46.66% <100.00%> (ø)
...e/Configuration/ResourceDescriptorAssemblyCache.cs	100.00% <100.00%> (ø)
...piDotNetCore/Configuration/ResourceGraphBuilder.cs	95.45% <100.00%> (ø)
...tCore/Configuration/ServiceCollectionExtensions.cs	100.00% <100.00%> (ø)
... and 52 more

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[bart-degreed]

Bug for suppressed warning on HttpRequest.ContentType in JsonApiMiddleware.ValidateContentTypeHeaderAsync tracked at: dotnet/aspnetcore#32097.