leap-seconds: Update to leap-seconds.3960835200 from IERS

Current leap-seconds file has already expired on 28 June 2025

PR: 289352
Obtained from: https://hpiers.obspm.fr/iers/bul/bulc/ntp/leap-seconds.3960835200

(cherry picked from commit 4729d5e2e2e3013fe1aaf92d5d932a3414f22ab3)