build: go back to dune 2.0 (the issue I was trying to fix was due to opam)

3618a518 · erwan · 6317b407 · 3618a518
Commit 3618a518 authored Mar 02, 2022 by erwan
--- a/dune-project
+++ b/dune-project
-(lang dune 3.0)
+(lang dune 2.0)