refactor(elixir/phoenix): change to use Config over Mix.Config (#7608)

* refactor: change to use Config over Mix.Config

Fixes the deprecated warnings to use Config.
See https://hexdocs.pm/mix/1.12/Mix.Config.html

* fixup! refactor: change to use Config over Mix.Config

frameworks/Elixir/phoenix/config/config.exs

@@ -3,7 +3,7 @@
 #
 # This configuration file is loaded before any dependency and
 # is restricted to this project.
-use Mix.Config
+import Config
 
 config :phoenix, :json_library, Jason
 

frameworks/Elixir/phoenix/config/dev.exs

@@ -1,4 +1,4 @@
-use Mix.Config
+import Config
 
 # For development, we disable any cache and enable
 # debugging and code reloading.

frameworks/Elixir/phoenix/config/prod.exs

@@ -1,4 +1,4 @@
-use Mix.Config
+import Config
 
 config :hello, HelloWeb.Endpoint,
   url: [host: "0.0.0.0"],

frameworks/Elixir/phoenix/config/test.exs

@@ -1 +1 @@
-use Mix.Config
+import Config