Part of WT-18574. Final source ticket: with the tests (WT-18872) and the source strip (WT-18874) merged, we now the core.

Work:

• move __wt_blkcache_open to block_mgr.c, name and behavior unchanged, closes WT-14642 as Fixed
• delete block_cache.c/.h, the WT_BLKCACHE connection state, WT_VERB_BLKCACHE (repoint the block-close log line to WT_VERB_BLOCK) and the whole memkind build
• keep block_io.c, block_map.c, block_mgr.c, the module/directory name and the victim-cache counters (disagg_victim_cache_* since WT-18764)
• add the upgrading.dox note: enabled=true in the API string, WiredTiger.config or the environment errors, disabled settings warn and are ignored, a persisted block_cache setting in the base configuration file is ignored with a warning - no override needed, no migration
• fix stale references: the wiredtiger.h.in page-log comments say "victim cache", the blkcache_exiting example in hardware.h

Definition of done:

• no WT_BLKCACHE references remain
• everything regenerated (dist/s_all, dist/s_docs)
• builds with and without memkind installed never link it.