LettuceEncrypt is now archived

The [docs](https://learn.microsoft.com/en-gb/aspnet/core/fundamentals/servers/yarp/lets-encrypt?view=aspnetcore-9.0) for Let's Encrypt mention using [natemcmaster/LettuceEncrypt ](https://github.com/natemcmaster/LettuceEncrypt). As it happens, though, the repository for it was archived on 24th of April


with the [explanation](https://github.com/natemcmaster/LettuceEncrypt/issues/307) being that it was a solo-maintained project with little activity and progress.

What does that mean for YARP support for Let's Encrypt? LettuceEncrypt was archived only recently, so it's sure to work still for the foreseeable future, but it's also bound to stop working *eventually*.