Where possible, use built-in JSON instead of Jason

Note that it's not possible to replace any call where we encode and
pretty print, as JSON doesn't have such functionality.

lib/music_brainz/api_impl.ex

@@ -439,7 +439,7 @@ defmodule MusicBrainz.APIImpl do
 
     case Finch.request(req, MusicBrainz.Finch) do
       {:ok, response} when response.status == 200 ->
-        {:ok, Jason.decode!(response.body)}
+        {:ok, JSON.decode!(response.body)}
 
       {:ok, response} when response.status in 301..308 ->
         location = :proplists.get_value("location", response.headers)