bug#74155: upcasing strings doesn’t respect standard-case-table

["Thomas Voss" via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi all, quick bug report.

As of 2017 (I believe), the capital eszett (ẞ) was adopted into the
German alphabet as the uppercase variable of ß which was previously (and
which still can be) uppercased to ‘SS’.  Since I prefer to use the newer
ẞ to the older SS, I have the following line in my configuration:

	(set-case-syntax-pair ?ẞ ?ß (standard-case-table))

When working with characters, this behaves as intended:

	(upcase ?ß)
	⇒ ?ẞ

However when working with strings, it doesn’t:

	(upcase "ß")
	⇒ "SS"

The same goes for the ‘upcase-word’ and ‘upcase-dwim’ functions which
still upcase ß to SS.  It seems that whatever code that is handling
case-conversions for multi-character inputs is not respecting the current
case table.

— Thomas