Address tzdata breaking changes

[Bisaloo]

As reported by CRAN:

Dear maintainer,

Please see the problems shown on https://cran.r-project.org/web/checks/check_results_lightr.html.

The errors on the Debian check systems are from a recent system upgrade
to tzdata 2024b which did

   Names present only for compatibility with UNIX System V
   (last released in the 1990s) have been moved to 'backward'.

which includes CET, CST6CDT, EET, EST*, HST, MET, MST*, PST8PDT, and
WET.

Debian ships the names in 'backward' in a separate package tzdata-legacy
which is not installed "by default".

The comments in the tzdata 'backward' file say

# Although this file is optional and tzdb will work if you omit it by
# building with 'make BACKWARD=', in practice downstream users
# typically use this file for backward compatibility.

so clearly one cannot unconditionally assume that the backward
compatibility names will work.

To be fixed before 2024-12-09