Update samples

[stjepan]

I noticed some of your samples aren't updated/correctly setup. You should update them to at least cover minimum supported target versions, which are .net core 2.1 and .net framework 4.6.1

This is how it looks today and some suggestions:

samples/UsageSample.AspNetCore

• targets .net core 2.1

samples/UsageSample.BearerTokenTester

• targets .net 4.5, but should be updated for .net 4.6.1

samples/ServerTester

• targets .net 4.6.1, but is not correctly configured:
• App.config
  change sku to .NETFramework,Version=v4.6.1
  

samples/UsageSample

• targets .net 4.6.1, but is not correctly configured:
• Web.config
  change both targetFramework to 4.6.1
  
  

samples/UsageSampleMvc.AspNetCore

• targets .net core 2.2