CI: Bump lint stage to elixir-1.12

Elixir 1.12 changed formatting rules, this allows to avoid having to rollback to run `mix format`

lib/pleroma/web/activity_pub/mrf.ex

@@ -157,9 +157,7 @@ defmodule Pleroma.Web.ActivityPub.MRF do
           [description | acc]
         else
           Logger.warn(
-            "#{policy} config description doesn't have one or all required keys #{
-              inspect(@required_description_keys)
-            }"
+            "#{policy} config description doesn't have one or all required keys #{inspect(@required_description_keys)}"
           )
 
           acc