Log level not compliant with rfc5424

Hello,

Following https://github.com/Seldaek/monolog/blob/main/doc/01-usage.md#log-levels:

There is strange level for each severity especially if it's supposed to be compliant with rfc5424.

In fact, it's not really the case. According to https://datatracker.ietf.org/doc/html/rfc5424#section-6.2.1 (table 2): "Severity values MUST be in the range of 0 to 7 inclusive."

Could you please change it?

It can lead to some issues during parsing from some log parsers ootb.

Thank you!