Fix compiler warnings

[ollien]

This fixes some long-standing compiler warnings in this repo

1. Migrating off Mix.Config. This was just a matter of following these steps
2. Using Application.compile_env/3 instead of Application.get_env/3
3. Using Logger.warning instead of Logger.warn
4. Bumps dev dependencies to more modern versions (as these versions emit compiler warnings).

This all required bumping the required Elixir version, but 1.5 is almost ten years old at this point, and 1.15 is almost three, so I think this is a reasonable bump :)

[sanchda]

👍