[9.18] fix: dev: Use attach()/detach() functions instead of touching .references

In rbtdb.c, there were two places where the code touched .references
directly instead of using the helper functions.  Use the helper
functions instead.

Forward port from https://gitlab.isc.org/isc-private/bind9/-/merge_requests/753

Merge branch 'ondrej/use-attach-detach-in-rbtdb-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9796