Merge branch '4.4' into 5.0

* 4.4:
  minor #37141 [Messenger] fix forward compatibility with Doctrine DBAL 2.11+ (xabbuh)
  fix forward compatibility with Doctrine DBAL 3
  [WebProfilerBundle] Set NullLogger for functional tests
  [Mailer] Fixed generator bug when creating multiple transports using Transport::fromDsn
  [FrameworkBundle] Fix XSD definition
  Update welcome.html.php
  [travis] add nightly to allowed failures

Author: nicolas-grekas

Resources/welcome.html.php

@@ -65,7 +65,7 @@
 <div class="wrapper">
     <div class="warning">
         <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 576 512" width="32"><path fill="currentColor" d="M569.517 440.013C587.975 472.007 564.806 512 527.94 512H48.054c-36.937 0-59.999-40.055-41.577-71.987L246.423 23.985c18.467-32.009 64.72-31.951 83.154 0l239.94 416.028zM288 354c-25.405 0-46 20.595-46 46s20.595 46 46 46 46-20.595 46-46-20.595-46-46-46zm-43.673-165.346l7.418 136c.347 6.364 5.609 11.346 11.982 11.346h48.546c6.373 0 11.635-4.982 11.982-11.346l7.418-136c.375-6.874-5.098-12.654-11.982-12.654h-63.383c-6.884 0-12.356 5.78-11.981 12.654z" class=""></path></svg>
-        You're seeing this page because you haven't configured any homepage URL.
+        You're seeing this page because you haven't configured any homepage URL and <a href="https://symfony.com/doc/<?= $docVersion; ?>/debug-mode" style="padding: 0 7px">debug mode</a> is enabled.
     </div>
 
     <div class="container">