Refactor sample apps for .NET 6 in-proc/out-of-proc

[justinyoo]

This PR is:

• To remove all existing sample apps targeting v2 - .NET Core 2.1.
• To remove all existing sample apps targeting v3 - .NET Core 3.1 and .NET 5.
• To add new sample apps targeting v4 - .NET 6 in-proc and out-of-proc workers.
  • New sample app uses the Pet Store example from https://editor.swagger.io.