web endpoint: Use Config.get directly instead of a tuple

Fixes a lot of warnings like the following while running the testsuite:

  warning: passing a {module, function, args} tuple to Plug.Parsers.MULTIPART is deprecated. Please see Plug.Parsers.MULTIPART module docs for better approaches to configuration

This might mean no more dynamic configuration but there seems to be the same limitation two lines underneath anyway.

lib/pleroma/web/endpoint.ex

@@ -102,7 +102,7 @@ defmodule Pleroma.Web.Endpoint do
   plug(Plug.Parsers,
     parsers: [
       :urlencoded,
-      {:multipart, length: {Config, :get, [[:instance, :upload_limit]]}},
+      {:multipart, length: Config.get([:instance, :upload_limit])},
       :json
     ],
     pass: ["*/*"],