Fix compilation warnings on recent versions of Elixir

[axelson]

I fixed all the compilation warnings on Elixir 1.16

Notes:

• Config was added in 1.9 (released 5 years ago) https://elixir-lang.org/blog/2019/06/24/elixir-v1-9-0-released/
• sigil_c was added pre-1.0

The warnings were:

> mix compile --warnings-as-errors
    warning: use Mix.Config is deprecated. Use the Config module instead
    │
  3 │ use Mix.Config
    │ ~~~~~~~~~~~~~~
    │
    └─ config/config.exs:3

Compiling 3 files (.ex)
     warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead
     │
 266 │   if :erlang.system_info(:otp_release) >= '18' do
     │                                           ~
     │
     └─ lib/temp.ex:266:43