Update to ASP.NET Core 3.1

[loic-sharma]

⚠️ This is still a work in progress!

Updates BaGet to ASP.NET Core 3.1. This is based off @MarkZither's work (see #443). Thank you for all the help @MarkZither!

TODO:

• Fix The property ... on entity type ... is a collection or enumeration type with a value converter but with no value comparer. warnings. See Use correct value comparers for collection properties with value conversion dotnet/efcore#17471 (comment)
• ⚠️ SPA proxy to React development server throws exceptions incessantly. See SPA proxy to React development server throws dotnet/aspnetcore#18470

Test plan

• Databases
  • SQLite
  • MySQL
  • PostgreSQL
  • SQL Server
• Storage
  • Azure
  • Filesystem
  • GCP
  • AWS
• Docker
• GitHub release
• Clients
  • nuget.exe
  • dotnet CLI
  • Visual Studio

Part of #439

[loic-sharma]

⚠️ SPA proxy to React development server throws exceptions incessantly. See dotnet/aspnetcore#18470

This seems to be an ASP.NET Core 3 regression, so I'll move forward with this change.