[tz] C++20 time library

[Paul Eggert]

One other thing. Internet RFC 8536 discusses TZif file truncation, and 
says that TZif files should not be used to convert timestamps outside of 
the truncated range. This discussion is improved in the latest draft for 
RFC 8536's successor, here:

https://datatracker.ietf.org/doc/draft-murchison-rfc8536bis/

which also talks about expiration of leap second tables.

Should the C++20 time library address the issues of TZif truncation and 
leap second table expiration, or treat these issues as being out of 
scope? Either way, some text would be advisable.