Partly fixes <https://bugs.gnu.org/28159>.
Suggested by Hartmut Goebel <h.goebel@crazy-compilers.com>.
* guix/gnu-maintenance.scm (%gnu-file-list-uri): New variable.
(ftp.gnu.org-files, latest-gnu-release): New procedures.
(%gnu-updater)[pred]: Change to GNU-HOSTED?.
[latest]: Change to LATEST-GNU-RELEASE.
(%gnu-ftp-updater): New variable.