README and sample updates to match recent .NET hosting models

[nblumhardt]

Fixes #368

• In all app models, UseSerilog is replaced with IServiceCollection.AddSerilog() - this works with WebApplicationBuilder and IHostApplicationBuilder (generic host).
• Removed writeToProviders from the README as there are enough breadcrumbs out there for people to find if they really need this, it's only required in very rare and undesirable scenarios.
• Blanket suppression of Microsoft.AspNetCore sources is no longer recommended; switched this for a handful of more targeted suppressions

[nblumhardt]

Not too much to review here, I'll send this through as the current README's looking pretty outdated 😅

[jcasale]

I think the comment in the sample is out of date, it still references UseSerilog().

[nblumhardt]

Fixed, thanks @jcasale