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

.stylelintrc.js

@@ -17,7 +17,7 @@ module.exports = {
         "at-rule-no-unknown": null,
         "no-descending-specificity": null,
         "scss/at-rule-no-unknown": [true, {
-            // https://github.com/vector-im/riot-web/issues/10544
+            // https://github.com/vector-im/element-web/issues/10544
             "ignoreAtRules": ["define-mixin"],
         }],
     }