rc.conf: correct $ntp_leapfile_sources

IETF is no longer serving leap-seconds.list. Update to the canonical place.

This fixes "service ntpd fetch".

IERS is the source of truth for leap seconds. Their leapsecond file is
updated most quickly and is always right (unlike the IANA one which
often lags). IERS operates this public service for the express purpose
of random people downloading it. Their terms of service are compatible
with open source (we could include this in our release). Rather than
fighting with questions around this because the IANA one changed
locations or the auto update script broken, just use this.

This is in preference to the NIST ftp copy. NIST is in the process of
retiring their FTP services.

Sponsored by: Netflix
Reviewed by: philip, delphij, cy
Differential Revision: https://reviews.freebsd.org/D43752

(cherry picked from commit b1c95af45488bef649e9a84890e2414ff80b3a00)
(cherry picked from commit 163c4342381530cfc5dc78a457cb1f9e355d687a)
(cherry picked from commit 11da791920ba285f0832f09cb504ac81e35ff8d1)
(cherry picked from commit ded562d4af9c1a7c6ea48271cbf07a81d7e031c4)

Security: FreeBSD-EN-24:01.tzdata
Approved by: so (gordon)