Fix external link icons on DuckCon 7 buttons

Replace <button><a> nesting with semantic <a class="button"> elements.

Author: Franz-Kafka

_events/2026-05-05-duckcon7.html

@@ -193,8 +193,8 @@ <h3 class="title-1">{{ venue_access.title }}</h3>
 <section class="join-us">
   <div class="card max-width-small">
     {% include duckcon7/section_header.html title="Join us for DuckCon #7!" description="The Call for Talks and Registration for DuckCon #7 are now officially open!" %}
-    <button class="button yellow"><a href="https://forms.gle/Lh6Zh96dJRi2PZEH8" style="color: black">Submit Your Talk</a></button>
+    <a class="button yellow" href="https://forms.gle/Lh6Zh96dJRi2PZEH8" target="_blank">Submit Your Talk</a>
     &nbsp;&nbsp;&nbsp;&nbsp;
-    <button class="button yellow"><a href="https://luma.com/3hi3m00x" style="color: black">Register</a></button>
+    <a class="button yellow" href="https://luma.com/3hi3m00x" target="_blank">Register</a>
   </div>
 </section>