fix: dev: Fix memory leak in dns_catz_options_setdefault() for zonedir

When defaults->zonedir is set, opts->zonedir is unconditionally
overwritten without freeing the previous value. This leaks memory
on every catalog zone update when zonedir defaults are configured.

Free the existing opts->zonedir before replacing it.

Merge branch 'ondrej/fix-memory-leak-in-dns_catz_options_setdefault' into 'main'

See merge request isc-projects/bind9!11660