docs(docs-infra): fix v21 event video not responsive on mobile

Replace raw iframe with docs-video component to fix YouTube embed
overflowing on mobile viewports.

Author: erkamyaman

adev/src/content/events/v21.md

@@ -22,16 +22,4 @@ Angular v21 is being delivered to you as a brand new release adventure. With mod
 - Your first look at Signal Forms, our new streamlined, signal-based approach to forms in Angular
 - Exciting new details about the Angular Aria package
 
-<div style="display: block; width: 80%; margin: 0 auto; margin-top: 20px;">
-  <iframe
-    credentialless
-    width="560"
-    height="315"
-    src="https://www.youtube.com/embed/DDAHORVzQ5g?si=B9Uv5vXzIKJfcLGr"
-    title="YouTube video player"
-    frameborder="0"
-    allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
-    referrerpolicy="strict-origin-when-cross-origin"
-    allowfullscreen
-  ></iframe>
-</div>
+<docs-video src="https://www.youtube.com/embed/DDAHORVzQ5g?si=B9Uv5vXzIKJfcLGr" title="Angular v21 Developer Event"/>