Analyzer: Warn that UseStartup/Configure and ConfigureWebHost isn't supported on WebApplicationBuilder

[davidfowl]

Today we throw in the following situations:

var builder = WebApplication.CreateBuilder(args);
builder.Host.ConfigureWebHost(builder =>
{

});

var builder = WebApplication.CreateBuilder(args);

// These should all fail with errors as well
builder.WebHost.UseStartup<Startup>();
builder.WebHost.UseStartup(typeof(Startup));
builder.WebHost.UseStartup("Startup");
builder.WebHost.UseStartup(context => new Startup());

var builder = WebApplication.CreateBuilder(args);
builder.WebHost.Configure(app =>
{

});

We should add an analyzer that detects these patterns that warns/fails and tells the user to configure the pipeline on WebApplication itself.

[martincostello]

@davidfowl If you'd like to assign this one to me I'll see if I can get it working.

[martincostello]

First analyzers attempt in #35884 - be kind 😄