Set the effective log severity level
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.