Add update hook for minimum severity level configuration

Problem/Motivation

Version 3.0 added a configurable minimum severity level, but did not add an update hook to set the default value so nothing would be logged as $this->config->get('severity_level') is NULL:

    if ($level > $this->config->get('severity_level')) {
      // Severity level is above the ones we want to log.
      return;
    }

Steps to reproduce

Install v 8.x-1.5 then update to 3.0.0

Proposed resolution

Add update hook to set the severity_level config to 3, as per the /config/install/log_stdout.settings.yml file.

User interface changes

None.

API changes

None.

Data model changes

None.