Obsolete Kestrel's Libuv transport

[halter73]

Internal Review

Fixes dotnet/aspnetcore#26245

[guardrex]

I'll edit and merge first thing Thursday morning (4am ... 😢 ... when the 😼 wakes me up!).

[guardrex]

There's a minor concern on the headings. Recently, the docs build system has been throwing warnings for headings duplicated across versions because rendering them produces heading tags with identical id values (e.g., <h2 id="libuv-transport-configuration">). We've been using a few approaches to work around this. However, the build thus far 🤞 isn't flaking out. Let's see how it goes. If I notice that this PR leads to warnings later, I'll follow this up with a PR to fix heading warnings.

[Pilchie]

Thanks @halter73!