Modify announcement banner style and message

Updated the announcement banner's font size and content for clarity.

Author: mkutmon

_includes/header.html

@@ -47,7 +47,7 @@
     </nav>
   </div>
   <!-- ANNOUNCEMENT BANNER -->
-<div style="background-color: #fff3cd; color: #333; text-align: center; padding: 10px 20px; font-size: small; border-bottom: 1px solid #f0ad4e;">
+<div style="background-color: #fff3cd; color: #333; text-align: center; padding: 10px 20px; font-size: medium; border-bottom: 1px solid #f0ad4e;">
   ⚠️ <strong>Transition update:</strong><br/>
   As previously announced, the Classic WikiPathways website is now read-only, and the old web service has been turned off. This does not affect the live website and is a major milestone in our transition to the new GitHub-based system! From 1 May 2026, the Classic site will be fully retired.<br/>
   <strong>Webservice users:</strong> If you are still using the old web service, please <a href="https://github.com/wikipathways/wikipathways-help/discussions" style="color: #333; text-decoration: underline;">contact us immediately</a> so we can help you transition.<br/>