[9.20] fix: usr: Nested DNS validation could cause assertion failure

When multiple nested DNS validations were destroyed out of order,
the EDE context could be freed before all EDE codes were copied,
which could cause an assertion failure. This has been fixed.

Closes #5213

Backport of MR !10365

Merge branch 'backport-5213-use-dns_ede_copy-in-dns_validator-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10366