chg: usr: Switch to LRU-only cache eviction, enforce minimum cache size

Busy resolvers will now gradually fill the configured :any:max-cache-size
before entries start being evicted. Previously, expired records were
proactively removed based on their TTL, which kept memory usage below the
configured limit but added overhead. Cache eviction now relies solely on the
SIEVE-LRU mechanism, which has matured to the point where TTL-based cleaning
is no longer necessary.

Setting :any:max-cache-size to unlimited or 0 is no longer supported
and falls back to the default (90% of physical memory).

Merge branch 'ondrej/no-ttl-based-cleaning' into 'main'

See merge request isc-projects/bind9!11459