pgbouncer: increase file descriptor limit
    
    With the current allowed connection count, PgBouncer would run out of file
    descriptors if under load. It does not raise the soft limit on its own.
    Note according to systemd.exec(5) this option should not be used. But it
    does give an alternative to raising the limit, and it's common practice
    to use it for processes run under systemd.
    
    https://progress.opensuse.org/issues/182522
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>