Prevent 503s from IIS after IISNativeApplication gets finalized

[halter73]

This was brought to my attention by dotnet/AspNetCore.Docs#34492, but @Tratcher had already filed an issue for it. Both of these are based on the investigation at https://techcommunity.microsoft.com/blog/iis-support-blog/asp-net-core-503-server-has-been-shutdown/3830338.

@Tratcher's suggestion was to detect this situation and provide a better error, but so long as we haven't actually started the server, I don't think this needs to be an error case at all. We could consider making the server restartable, but I think that would require more drastic changes, and that's not something people try to do often.

What's more common is code like the following:

var config = WebApplication.CreateBuilder().Configuration;
var someConfig = config["someConfig"];

While I would never recommend this, and would advocate for creating your own ConfigurationManager or ConfigurationBuilder from the appropriate source instead, this should not break IIS.

Fixes #48457