Add `<s>` to `$allowedtags` in KSES.

From https://developer.mozilla.org/en-US/docs/Web/HTML/Element/s :
"The <strike> element, alter ego of the <s> element is obsolete and should not be used on Web sites any more."

`<strike>` shall remain for BC, but `<s>` should not be stripped from the author (et al) role's HTML input.

Props paulschreiber.
Fixes #30954.


git-svn-id: https://develop.svn.wordpress.org/trunk@31205 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/kses.php

@@ -441,6 +441,7 @@ if ( ! CUSTOM_TAGS ) {
 		'q' => array(
 			'cite' => true,
 		),
+		's' => array(),
 		'strike' => array(),
 		'strong' => array(),
 	);