The "unstable" alert in OCaml 5.0 is enabled by default in merlin

The `Warnings` module in OCaml 5.0 doesn't correspond to what's in this module upstream:

https://github.com/ocaml/merlin/blob/c447386d83522b26eb359d489eb6f5b7488203a5/src/ocaml/utils/warnings.ml#L507

Upstream, the `unstable` alert is disabled by default:

https://github.com/ocaml/ocaml/blob/34cf5aafcedc2f7895c7f5f0ac27c7e58e4f4adf/utils/warnings.ml#L861