CI: Bump lint stage to elixir-1.12

Elixir 1.12 changed formatting rules, this allows to avoid having to rollback to run mix format, that said it also means contributors will have to upgrade, a noticeable distro lacking 1.12.0+ is Debian/Ubuntu.

Quite makes me wish the rules could/would be specified in the config (like with clang-format for other languages) rather than version-dependent.