Require Elixir 1.13

This was instigated by credo not working on Elixir 1.11 any more. Other
libraries are requiring Elixir 1.13 and later now anyway, so this
probably doesn't affect anyone in practice.

.circleci/config.yml

@@ -15,9 +15,7 @@ tags: &tags
     1.15.7-erlang-26.1.2-alpine-3.18.4,
     1.14.5-erlang-25.3.2-alpine-3.18.0,
     1.13.4-erlang-25.3.2-alpine-3.18.0,
-    1.13.4-erlang-24.3.4.11-alpine-3.18.0,
-    1.12.3-erlang-24.3.4.11-alpine-3.18.0,
-    1.11.4-erlang-23.3.4.18-alpine-3.16.2
+    1.13.4-erlang-24.3.4.11-alpine-3.18.0
   ]
 
 jobs:

mix.exs

@@ -8,7 +8,7 @@ defmodule NervesSSH.MixProject do
     [
       app: :nerves_ssh,
       version: @version,
-      elixir: "~> 1.11",
+      elixir: "~> 1.13",
       start_permanent: Mix.env() == :prod,
       deps: deps(),
       description: description(),