Bump all dependencies and add elixir 1.16 to CircleCI

[ConnorRigby]

Upgraded:
  cowboy 2.10.0 => 2.12.0
  cowlib 2.12.1 => 2.13.0
  credo 1.7.5 => 1.7.6
  elixir_make 0.7.8 => 0.8.4 (minor)
  ex_doc 0.31.1 => 0.34.0 (minor)
  makeup 1.1.1 => 1.1.2
  makeup_elixir 0.16.1 => 0.16.2
  makeup_erlang 0.1.3 => 1.0.0 (major)
  muontrap 1.4.0 => 1.5.0
  plug 1.15.3 => 1.16.0
  plug_cowboy 2.7.0 => 2.7.1
  plug_crypto 2.0.0 => 2.1.0
  vintage_net_wifi 0.12.3 => 0.12.5