Replace deprecated use Mix.Config with import Config (#918)

The `use Mix.Config` method has been deprecated. The new `import Config`
method has been available since Elixir 1.9, which is the oldest version
we support.

> The [Config](https://hexdocs.pm/elixir/main/Config.html#content)
> module in Elixir was introduced in v1.9 as a replacement to use
> Mix.Config, which was specific to Mix and has been deprecated.

https://hexdocs.pm/elixir/main/Config.html

---------

Co-authored-by: Tom de Bruijn <tom@tomdebruijn.com>

.changesets/update-mix-config-import.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "change"
+---
+
+Update the Mix config import in the `config/appsignal.exs` file to use `import Config`, rather than the deprecated `use Mix.Config`.

lib/mix/tasks/appsignal.install.ex

@@ -207,7 +207,7 @@ defmodule Mix.Tasks.Appsignal.Install do
         end
 
       true ->
-        case File.write(config_file_path(), "use Mix.Config\n#{active_content}") do
+        case File.write(config_file_path(), "import Config\n#{active_content}") do
           :ok ->
             IO.puts("Success!")
 
@@ -246,7 +246,7 @@ defmodule Mix.Tasks.Appsignal.Install do
       end
 
     """
-    use Mix.Config
+    import Config
 
     config :appsignal, :config,
     #{options_with_active}

test/mix/tasks/appsignal_install_test.exs

@@ -185,7 +185,7 @@ defmodule Mix.Tasks.Appsignal.InstallTest do
 
       assert String.contains?(
                appsignal_config,
-               ~s(use Mix.Config\n\n) <>
+               ~s(import Config\n\n) <>
                  ~s(config :appsignal, :config,\n) <>
                  ~s(  otp_app: :appsignal,\n) <>
                  ~s(  name: "AppSignal test suite app",\n) <>
@@ -224,7 +224,7 @@ defmodule Mix.Tasks.Appsignal.InstallTest do
 
       assert String.contains?(
                appsignal_config,
-               ~s(use Mix.Config\n\n) <>
+               ~s(import Config\n\n) <>
                  ~s(config :appsignal, :config,\n) <>
                  ~s(  active: true,\n) <>
                  ~s(  otp_app: :appsignal,\n) <>
@@ -262,7 +262,7 @@ defmodule Mix.Tasks.Appsignal.InstallTest do
 
       assert String.contains?(
                File.read!(Path.join(@test_config_directory, "config.exs")),
-               ~s(use Mix.Config\n\nimport_config "appsignal.exs")
+               ~s(import Config\n\nimport_config "appsignal.exs")
              )
     end
 
@@ -271,7 +271,7 @@ defmodule Mix.Tasks.Appsignal.InstallTest do
       @test_config_directory
       |> Path.join("config.exs")
       |> File.open!([:write])
-      |> IO.binwrite(~s(use Mix.Config\n# config\nimport_config "appsignal.exs"))
+      |> IO.binwrite(~s(import Config\n# config\nimport_config "appsignal.exs"))
       |> File.close()
 
       @test_config_directory
@@ -339,7 +339,7 @@ defmodule Mix.Tasks.Appsignal.InstallTest do
     directory
     |> Path.join("#{env}.exs")
     |> File.open!([:write])
-    |> IO.binwrite("use Mix.Config\n# #{env}")
+    |> IO.binwrite("import Config\n# #{env}")
     |> File.close()
   end
 
@@ -348,7 +348,7 @@ defmodule Mix.Tasks.Appsignal.InstallTest do
   defp config_active_for_env?(env) do
     case File.read(Path.join(@test_config_directory, "#{env}.exs")) do
       {:ok, env_config} ->
-        String.contains?(env_config, ~s(use Mix.Config\n# #{env}\n)) &&
+        String.contains?(env_config, ~s(import Config\n# #{env}\n)) &&
           String.contains?(env_config, ~s(\nconfig :appsignal, :config, active: true))
 
       {:error, _} ->