Warn about changing a version slug

Ref readthedocs/readthedocs.org#11930

Author: stsewd

readthedocsext/theme/templates/projects/version_form.html

@@ -53,7 +53,7 @@ <h2 class="ui medium dividing header">
           <div class="content">
             {% if version.active %}
               <div class="ui warning message">
-                {% trans "Deactivating a version will result in the deletion of its documentation." %}
+                {% trans "Deactivating a version or changing its slug will result in the deletion of its documentation." %}
               </div>
             {% endif %}