time-stamp: properly abbreviate instead of truncating names

time-stamp: properly abbreviate instead of truncating names

* lisp/time-stamp (time-stamp-string-preprocess): Stop truncating month
and weekday name strings; it didn't internationalize well.
Some historical conversions, previously accepted quietly, now warn.
(time-stamp-format): Recommend the simpler formats implemented in 2019.
* test/lisp/time-stamp-tests.el: Update tests and comments to match.
Revert commit 83e4559664 (2022-07-01), which was working around the
former confusion between truncation and abbreviation.

(cherry picked from commit 7665ec8df8d1c01211db6a85dda4813d6912ffee)