Tighten version constraints in opam files
Added these constraints to all opam files:
"ocaml" { >= "4.12.0" }
"dune" { >= "2.8" }
The repo uses language features introduced in OCaml 4.12.0. The constraint on dune is now consistent with dune-project
.
These errors were picked up by the opam CI and are already fixed for the opam files of published packages.