Update samples and README to use recent .NET host APIs

[nblumhardt]

Fixes #76

[ite-jankl]

Why does the README sample use CreateLogger while the SimpleServiceSample uses CreateBootstrapLogger?

[nblumhardt]

Hi! This post should explain it: https://nblumhardt.com/2020/10/bootstrap-logger/

[ite-jankl]

I don't see it?

It describes what the bootstrap logger is and how it works. It doesn't tell me why one example uses it while the other does not.

It also says a call to UseSerilog is necessary for the bootstrap logger to be migrated because that includes the migration magic. But neither example uses that - they use AddSerilog. (Both use CreateApplicationBuilder where #76 says there is and won't be a UseSerilog but to use AddSerilog instead).