[mailman] Disable processing of old bounce events

As mailman didn't processed bounce events, just saved them in the database, we
need to say that we don't want to process the old ones. See
https://docs.mailman3.org/en/latest/upgrade-guide.html#upgrade-from-3-3-1-to-3-3-1
for more info.

Signed-off-by: Michal Konecny <mkonecny@redhat.com>

roles/mailman/templates/mailman.cfg.j2

@@ -73,6 +73,8 @@ jump_chain: discard
 smtp_port: 587
 # Remove DKIM signatures in the email headers.
 remove_dkim_headers: yes
+# Don't process old stale bounces
+verp_probes: yes
 
 
 [language.en]