R1:df31405d64a4

Improve compatibility of "Config > Cache Status" across APCu versions

Summary:
Ref T13164. See PHI790. Older versions of APCu reported cache keys as "key" from `apcu_cache_info()`. APC and newer APCu report it as "info".

Check both indexes for compatibility.

Test Plan:
- Locally, with newer APCu, saw no behavioral change.
- Will double check on `admin`, which has an older APCu with the "key" behavior.
- (I hunted this down by dumping `apcu_cache_info()` on `admin` to see what was going on.)…