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/indexing/EventIndex.js

@@ -145,7 +145,7 @@ export default class EventIndex extends EventEmitter {
         const indexManager = PlatformPeg.get().getEventIndexingManager();
 
         if (prevState === "PREPARED" && state === "SYNCING") {
-            // If our indexer is empty we're most likely running Riot the
+            // If our indexer is empty we're most likely running Element the
             // first time with indexing support or running it with an
             // initial sync. Add checkpoints to crawl our encrypted rooms.
             const eventIndexWasEmpty = await indexManager.isEventIndexEmpty();