Drop compatibility with ocaml less than 4 14
requested to merge nomadic-labs/tezos:raphael-p@drop-compatibility-with-ocaml-less-than-4-14 into master
ocamlformat has an option to keep code compatible with previous versions of OCaml. Before this MR, the compatiblity mode was set to ocaml.4.12. This prevented the use of some modern syntactical constructs (e.g., let-punning in binding-operators and explicit naming in existentials). After this MR, ocamlformat is set to produce (and thus accept) ocaml.4.14.
Most of our opam files don't mention a lower-bound for ocaml, nor even a dependency to it. The MR also adds a dependency to ocaml >= 4.14 by default for all our packages (but leaves the possibility of setting a custom version constraint).