Also update README.md to explain to users (and future me).
Also update ci-bazel.yml to drop Python 3.8 and add 3.13.
(Missed this file when updating python.yml.)
Change-Id: Idbad8d392b88c5c38a59d5e0e2a1b5ff438990e6
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63792
Reviewed-by: Alan Donovan <adonovan@google.com>