[3.11] main/fail2ban: fix logging after rotate
fail2ban stops updating /var/log/fail2ban.log after rotation, this change fixes that problem.
According to the docs at: https://www.fail2ban.org/wiki/index.php/Commands#LOGGING we shall call "flushlogs" after rotation.
Closes #11382 (closed)
(cherry picked from commit 522044cc)