Add Level::toRFC5424Level method and restore BC to Syslog handlers (broken in #1689)

src/Monolog/Handler/AbstractSyslogHandler.php

@@ -40,6 +40,14 @@ abstract class AbstractSyslogHandler extends AbstractProcessingHandler
         'uucp'     => \LOG_UUCP,
     ];
 
+    /**
+     * Translates Monolog log levels to syslog log priorities.
+     */
+    protected function toSyslogPriority(Level $level): int
+    {
+        return $level->toRFC5424Level();
+    }
+
     /**
      * @param string|int $facility Either one of the names of the keys in $this->facilities, or a LOG_* facility constant
      */