fix: nil: simplify dns_dumpctx API

the functions dns_dumpctx_db() and dns_dumpctx_version() are used in
only one place, to get the serial number of the version being dumped.
it's simpler to expose the serial number through its own call,
dns_dumpctx_serial(), and remove the others.

Merge branch 'each-cleanup-dumpctx' into 'main'

See merge request isc-projects/bind9!10937