strike element is deprecated, s replaces it (#1629)

* strike element is deprecated, s replaces it

* Add newsfragment

changelogs/client_server/newsfragments/1629.clarification

@@ -0,0 +1 @@
+The [strike](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/strike) element is deprecated in the HTML spec. Clients should prefer [s](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/s) instead.

content/client-server-api/modules/instant_messaging.md

@@ -45,7 +45,7 @@ HTML injection, and similar attacks. The strongly suggested set of HTML
 tags to permit, denying the use and rendering of anything else, is:
 `font`, `del`, `h1`, `h2`, `h3`, `h4`, `h5`, `h6`, `blockquote`, `p`,
 `a`, `ul`, `ol`, `sup`, `sub`, `li`, `b`, `i`, `u`, `strong`, `em`,
-`strike`, `code`, `hr`, `br`, `div`, `table`, `thead`, `tbody`, `tr`,
+`s`, `code`, `hr`, `br`, `div`, `table`, `thead`, `tbody`, `tr`,
 `th`, `td`, `caption`, `pre`, `span`, `img`, `details`, `summary`.
 
 Not all attributes on those tags should be permitted as they may be