leap-seconds: Update to leap-seconds 3960057600 obtained from IERS

IERS is the canonical source of leap-seconds. IANA, NIST and USNO obtain
their leap-second updates from IERS.

This resolves an issue for IPv6-only hosts as IERS is not accessible
via IPv6, requiring IPv6-only host to rely on some other source,
above.

From this point forward we should fetch the file from IERS, the
organization responsible for deciding when to insert leap-seconds.

PR: 279413
Obtained from: https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.list
Discussed with: imp

(cherry picked from commit 04b4dcf04fc345b29b78e5061768091648b8ee2a)