minor #10590 Revert a change related to flash messages and sessions (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Revert a change related to flash messages and sessions

This was done a few days ago (e6ce2f4) but there's an ongoing discussion about it in Symfony (symfony/symfony#28804) and some people are complaining on slack about the changed code. So let's revert for now to the original code.

Commits
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f96118b Revert a change related to flash messages and sessions

Author: javiereguiluz

session/avoid_session_start.rst

@@ -29,7 +29,7 @@ access the flash messages:
 
 .. code-block:: html+twig
 
-    {% if app.request.method == 'POST' %}
+    {% if app.request.hasPreviousSession %}
         {% for message in app.flashes('notice') %}
             <div class="flash-notice">
                 {{ message }}