fix: dev: Remove holes in `dns_zoneflg_t` enum

The `dns_zoneflg_t` enum defined multiple possible flags for a zone, but
contains numerous holes (likely from flag removed in the past). This
fixes the holes, and use a bit-shift and decimal notation to make holes
easier to spot.

Merge branch 'colin/remove-zoneflag-holes' into 'main'

See merge request isc-projects/bind9!11189