chg: usr: Enforce bounds of prefetch configuration option

The prefetch configuration option now enforces boundaries. The configuration (including when using `named-checkconf`) now fails if the trigger (first value) is above 10, and if the eligibility (second optional value) isn't at least six seconds greater than the trigger value.

Merge branch 'colin/prefetch-enforcebounds' into 'main'

See merge request isc-projects/bind9!11243