gnu-maintenance: KDE updater no longer relies on FTP access.

Fetch the ls-lR.bz2 file list for download.kde.org, convert it into a list of
file paths and cache the list.

* guix/gnu-maintenance.scm (%kde-file-list-uri): New variable.
  (download.kde.org-files): New procedure.
  (latest-kde-release): Change to use DOWNLOAD.KDE.ORG-FILES and search
  for files in this list.