[9.20] fix: dev: Fix OpenSSL 4 compatibility issue when calling X509_get_subject_name()

Starting from OpenSSL 4 the the X509_get_subject_name() function
returns a 'const' pointer to a name instead of a regular pointer.
Duplicate the name before operating on it, then free it.

Closes #5807

Backport of MR !11676

Merge branch 'backport-5807-openssl-4-X509_get_subject_name-compat-fix-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11692