chg: usr: Lowercase the NSEC next owner name when signing

When building the NSEC rdata, lowercase the next owner name before
storing it in the Next Domain Name Field.

Note that this is not required according to RFC 6840, but since there
is inconsistency in the documents over time, having uppercase next
owner names in the NSEC records may cause validation failures if
validators are not following RFC 6840.

Closes #5702

Merge branch '5702-lowercase-nsec-next-owner-name' into 'main'

See merge request isc-projects/bind9!11442