[tz] No leap second on 2021-12-31

[Kerry Shetline]

What’s interesting is that the Earth is undergoing an odd span of time during which it has been spinning a bit faster instead of slowing down. If this keeps up, not only will it be a while until we see another leap second, but in a few years we might experience the first negative leap second, where a second is dropped from the end of the last minute just before 00:00 UTC. 

> Per IERS, Bulletin C 62 (2021-07-05)
> https://hpiers.obspm.fr/iers/bul/bulc/bulletinc.62
> 
> ?and forwarded below.
> 
> A patch updating leap-seconds.list from NIST is attached, and is installed
> in the development repository.
> 
> --
> Tim Parenti