chg: dev: Make the database ownercase modifiable only via addrdataset()

Simplify the implementation around the database ownercase.  Remove the
dns_rdataset_setownercase() implementation for the slabheaders and only
allow setting ownercase on rdatalists and rdatasets.  The ownercase in
the database can now be set only with dns_db_addrdataset() by passing
rdataset with correctly set ownercase.

Merge branch 'ondrej/make-database-ownercase-mostly-constant' into 'main'

See merge request isc-projects/bind9!10971