[php-src] Issue #11268: Allow CTE on mb_xxx functions

Issue: https://github.com/php/php-src/issues/11268 Author: mvorisek ### Description mb_xxx functions like mb_strtoupper are pure and repeatable excl. they depend on mb_internal_encoding. In 99% cases the internal encoding is UTF-8. Given this, I propose to make the multibyte functions CTE/cacheable a) as long as the encoding is specified or b) as long as the internal encoding is the same/UTF-8 as it was during opcaching. Other alternative could be to deprecate internal encoding in PHP 8.3 and drop it in favor of fixed UTF-8 in PHP 9.0.