Revert change to legacy calendar.html

Per review on boostorg#2445: keep the updated copy on v3 only.

Author: julioest

templates/calendar.html

@@ -10,7 +10,7 @@ <h1 class="text-3xl">Boost Calendar</h1>
   </div>
   <div class="container px-4 my-4 mx-auto">
     <div class="section-body">
-      <p>Stay up to date with Boost events, releases, and community happenings. Release-related entries are maintained by the Boost release managers.</p>
+      <p>Below is a community maintained calendar of Boost related events. The release managers try to keep the release related portion of the calendar up to date.</p>
       <iframe src="https://www.google.com/calendar/embed?src={{ boost_calendar }}&amp;ctz=America/Chicago" class="c1" width="100%" height="600" frameborder="0" scrolling="no"></iframe>
     </div>
   </div>