Drop support for OCaml < 4.08.0

[hannesm]

Hello,

there's quite some craft to support old OCaml releases. I'm wondering what is the reason to support old OCaml versions? Are there users thereof? Are they not fine to upgrade to a newer OCaml version? Or can't they just keep on using old Lwt releases?

Especially with OCaml 5.0.0 on the horizon, I propose to cut backwards compatibility to get rid of various dependencies on compatibility packages and code paths. What do you think? If you like the idea, I can propose a PR for this.

[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.

[hannesm]

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