CRM_Core_BAO_Cache - Deprecate getItems(), getItem(), setItem(), deleteGroup()

CRM_Core_BAO_Cache - Deprecate getItems(), getItem(), setItem(), deleteGroup()

These interfaces predate PSR-16 -- which is more flexible and complete.
PSR-16 supports TTLs, default-values, multi-key operations, etc.  PSR-16
drivers can be extended/decorated/replaced.  There are third-party
implementations of PSR-16.  And (personally) I find the code which consumes
PSR-16 to be more readable+writeable (e.g. `$cache->get($key)` vs
`CRM_Core_BAO_Cache::getItem($group, $item)`).

However, `CRM_Core_BAO_Cache` has been around forever.  I currently count
ten distinct cache-groups which rely on it (5 from `civicrm-core` and 5 from
`universe`). So we shouldn't remove it outright.