[9.20] fix: usr: Fix the default interface-interval from 60s to 60m

[9.20] fix: usr: Fix the default interface-interval from 60s to 60m

When the interface-interval parser was changed from uint32 parser to
duration parser, the default value stayed at plain number `60` which
now means 60 seconds instead of 60 minutes.  The documentation also
incorrectly states that the value is in minutes.  That has been fixed.

Closes #5246

Backport of MR !10281

Merge branch 'backport-5246-fix-default-interface-interval-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10679