fix: dev: Fix a memory leak when updating a zone with more than 32 DNSSEC keys

Applying changes to a signed zone — via DNS UPDATE or the inline-signing
raw-to-secure sync — leaked the surplus keys when the zone's key
directory held more than 32, slowly growing named's memory use.

Closes #6051

Merge branch '6051-find-zone-keys-key-leak' into 'main'

See merge request isc-projects/bind9!12328