Task: archive the 8.4, 8.5, 8.6 images after some proper announcement

cf. this [Zulip post](https://coq.zulipchat.com/#narrow/channel/237656-Coq-devs-.26-plugin-devs/topic/heads-up.20on.20a.20new.20lower.20bound.20in.20opam-repository/near/497630438)

Cc @Zimmi48 I open this issue just to keep track of the task, but it is not urgent,
and I'll wait for the green light of the Rocq team anyway.