"max-cache-size" is a no-op since BIND 9.19.16

Commit 4db15043 (part of !7873 (merged)) inadvertently turned the max-cache-size option into a no-op by removing the isc_mem_setwater() call from dns_cache_setcachesize().

This was not caught in testing as the current memory use limitation logic employed in named is not stable enough for low max-cache-size values (which is documented) and most tests use the implicit default of max-cache-size 90%;.

Shout-out to @esesterhenn for spotting this, thank you! 🏅