Bump symfony/polyfill-mbstring from 1.31.0 to 1.32.0 #5960

Job	Run time
PHP 8.4 - WP latest (experimental) (in random order)	3m 23s
PHP 8.4 - WP latest (with coverage)	7m 15s
PHP 7.4 - WP 6.6	2m 39s
PHP 8.1 - WP latest	2m 52s
PHP 8.3 - WP latest	2m 50s
PHP 8.2 - WP latest	3m 27s
PHP 8.0 - WP latest	3m 6s
PHP 8.4 - WP trunk (experimental)	4m 28s
	30m 0s

Provide feedback