fix(mastodon_api): use the Mastodon date format for marker updated_at

Ported from Akkoma 720785b4c.

Co-authored-by: Oneric <oneric@oneric.stub>

lib/pleroma/web/mastodon_api/views/marker_view.ex

@@ -5,13 +5,15 @@
 defmodule Pleroma.Web.MastodonAPI.MarkerView do
   use Pleroma.Web, :view
 
+  alias Pleroma.Web.CommonAPI.Utils
+
   def render("markers.json", %{markers: markers}) do
     Map.new(markers, fn m ->
       {m.timeline,
        %{
          last_read_id: m.last_read_id,
          version: m.lock_version,
-         updated_at: NaiveDateTime.to_iso8601(m.updated_at),
+         updated_at: Utils.to_masto_date(m.updated_at),
          pleroma: %{
            unread_count: m.unread_count
          }