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The IETF are no longer serving the leap-seconds.list file, see their message at https://www.ietf.org/timezones/data/leap-seconds.list. Instead, they suggest to use IANA as source instead, i.e., https://data.iana.org/time-zones/data/leap-seconds.list (which in turn used to be a copy from NIST, and nowadays is a copy from the IERS). Update ntpleapfetch and some documentation accordingly.