How to use a newer version of a dep?

While using elixir-ls with helix(editor) I get the following error:

This only happens when I open mix.exs

My system doesn’t have elixir installed, only elixir-ls in the helix config.
The project repo has a flake which has dependencies for elixir_1_14:

My guess is elixir-ls is build using pkgs.elixir which is 1.13.4
Should I change these from elixir to elixir_1_14?

I guess it might be better to also put elixir-ls inside my project repo instead of on the whole system?