Add `rsync://ftp.math.utah.edu/historic/` to historic mirrors
This MR updates scripts/pick-historic-mirror.sh
after almost two years.
As a side effect, scripts/pick-historic-mirror.sh 2023
now returns rsync://ftp.math.utah.edu/historic/
instead of rsync://mirrors.tuna.tsinghua.edu.cn/tex-historic-archive/
. Unlike the latter, the former already contains the sub-directory systems/texlive/2023/tlnet-final
. Therefore, after this MR, we can start building the texlive/texlive:TL2023-historic
images.