Replace Riot with Element in docs and comments

This only covers the simple cases of references to issues and repos. More
complex areas, such as deployment scripts, will be handled separately.

Part of https://github.com/vector-im/element-web/issues/14864

src/utils/pillify.js

@@ -139,7 +139,7 @@ export function pillifyLinks(nodes, mxEvent, pills) {
  * It's critical to call this after pillifyLinks, otherwise
  * Pills will leak, leaking entire DOM trees via the event
  * emitter on BaseAvatar as per
- * https://github.com/vector-im/riot-web/issues/12417
+ * https://github.com/vector-im/element-web/issues/12417
  *
  * @param {Node[]} pills - array of pill containers whose React
  *   components should be unmounted.