devel/ruby-zeitwerk: update to 2.6.13

2.6.13 (2024-02-06)

* There is a new experimental null inflector that simply returns its input
  unchanged:

    loader.inflector = Zeitwerk::NullInflector.new

  Projects using this inflector are expected to define their constants in
  files and directories with names exactly matching them:

    User.rb      -> User
    HTMLParser.rb -> HTMLParser
    Admin/Role.rb -> Admin::Role

  Please see its documentation for further details.

* Documentation improvements.

(taca)