Drop support for OCaml < 4.08.0 about lwt

[hannesm]

Thanks @raphael-proust for your survey, and your work on reducing the dependency cone. Also thanks for preparing the 5.6 releases :)

[raphael-proust]

I like the idea. I considered it in the past and I think that we should possibly run an informal poll on discuss just to give a chance for potential users to make themselves known.

[raphael-proust]

I've made https://discuss.ocaml.org/t/lwt-informal-user-survey/9666 with a series of polls about which OCaml versions are used and such.

Anyone who wishes to participate information but cannot or doesn't want to do it via discuss, do not hesitate to reply in this thread or send me a message through different means. The important questions are: What version of OCaml do you use Lwt with? What versions of Lwt do you use? What OS/platform do you use Lwt on?

[raphael-proust]

This is now done.