Set the effective log severity level In a Drupal 8 site, on the page /admin/config/development/logging, I can specify if I want warnings, errors and other messages to be displayed. But either I want them displayed or not, they exist.

On this other page, admin/reports/dblog , I can see all the logs available in database.

But if my site has many visitors, Debug, Info and Notice logs are constantly inserted in the database. Which causes high charge on the database and "hides" the important logs (errors and warnings). 

How can I tel Drupal "Do not insert in database any log with a severity level below WARNING" ? The equivalent of the level value we find in every logging framework.
There is no such setting unfortunately. There is a module for 7.x, but not 8.x port yet: https://www.drupal.org/project/watchdog_filtering.

You could use a different watchdog backend that supports filtering, or log to syslog or some other system that is better at dealing with a lot of log entries.

And if those notices/warnings are caused by PHP notices/warnings, then you should fix them.