Follow-up to WT-18876, which kept these names so its deletion diff stayed pure. These functions have shared block I/O now, nothing block-cache about them:

  __wt_blkcache_read, __wt_blkcache_read_multi, __wt_blkcache_write, __wt_blkcache_open,
  __wt_blkcache_compress, __wti_blkcache_map, __wti_blkcache_map_read, __wti_blkcache_unmap, and
  the block_io.c statics (__blkcache_read_corrupt, __blkcache_read_decrypt)

Rename them and update callers, no behaviour change. Consider whether the src/block_cache/ }}directory rename folds in too, and whether {{checkpoint_rec_blkcache_write and reconciliation's blkcache_write_time join in

Out of scope: {}wti_blkcache_user_config_check _and }}{{_conn_blkcache_base_config_warn. These check the block_cache configuration category, which keeps its name

Done when no blkcache name remains in src/ and dist/s_all is green.