[tz] proposal for new tzdb versions

[Florian Weimer]

* Paul Eggert via tz:

> * We also generate a separate 2021a1 version, which is like 2021a
>   except with the Samoa change that is prompting 2021b. This version
>  recognizes the concerns about the number of changes to pre-1970
> timestamps in 2021b. I'll do this by publishing a patch to 2021a,
> along with a patched tarball, on my website at UCLA.

I'm slightly worried that people have grown to depend on the \d+[a-z]+
format for version numbers, so this choice of version might break some
things.

Thanks,
Florian