gnu-maintenance: Exclude development releases from GNOME update candidates.

Suggested by Efraim Flashner <efraim@flashner.co.il>.

* guix/gnu-maintenance.scm (latest-ftp-release): Add #:keep-file?
parameter and honor it.
(latest-gnome-release)[%not-dot]: New variable.
[even-minor-version?, even-numbered-tarball?]: New procedures.
Pass EVEN-NUMBERED-TARBALL? as #:keep-file? argument.