chg: dev: Reduce the memory used by each record set in the cache

Each record set in the cache carried a 32-byte table for
restoring its owner name's letter case, unused since the
case handling was reworked. Removing it makes every cache
entry 32 bytes smaller.

Merge branch 'ondrej/remove-upper-from-slabheader' into 'main'

See merge request isc-projects/bind9!12478