fix: fix Elixir 1.15 depreciations

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/email/group.ex

@@ -70,7 +70,7 @@ defmodule Mobilizon.Web.Email.Group do
          %Actor{type: :Group, preferred_username: group_username},
          user
        ) do
-    Logger.warn(
+    Logger.warning(
       "Unable to notify group follower user #{user.email} for event #{event_uuid} from group #{group_username}"
     )