Add I/O register now banner (#5765)

Co-authored-by: Marya <111139605+MaryaBelanger@users.noreply.github.com>

Author: parlough

src/_includes/banner.html

@@ -1,9 +1,6 @@
 <div class="banner">
   <p class="banner__text">
-    Dart 3.3 is here!
-    Try new extension types, next-generation JavaScript interop, and more.
-    <br>
-    Read the
-    <a href="https://medium.com/dartlang/dart-3-3-325bf2bf6c13">blog post</a>
+    Dart and Flutter are back at Google I/O on May 14!
+    <a href="https://io.google/2024/?utm_source=flutter&utm_medium=embedded_marketing" target="_blank">Register now</a>
   </p>
 </div>