lang/elixir-devel: update to 1.15.0

- https://elixir-lang.org/blog/2023/06/19/elixir-v1-15-0-released/

Note that Elixir now *requires* OTP25+ or better at runtime.

For the -devel flavour you are still free to choose OTP25 or
OTP26, but the default OTP24 from lang/erlang will *not* work.