Periodical run of disabled special pages: make updates monthly

All looks well (<https://bugzilla.wikimedia.org/show_bug.cgi?id=15434#c79>):
proceed to step 2 (<https://bugzilla.wikimedia.org/show_bug.cgi?id=15434#c70>).
We have already experimented with one of the most expensive pages and update
didn't cause problems; making it monthly doesn't change substance of load impact.

Using "absent" to unset the month restriction per
<https://docs.puppetlabs.com/references/latest/type.html#cron>.

Bug: 15434
Change-Id: I52a4b8f1b427760a2a77acc4d4b48170617cb242