[10.x] Fix issue with ArrayCache

[driesvints]

This one seems to have snuck into #48573 by accident.

[driesvints]

@timacdonald the failing tests here tell me this was done deliberately probably?

[timacdonald]

The existing behaviour is consistent with Redis^ and is how it should work.

If you cache something at 2000-01-01 00:00:05 with a TTL of an hour, it should expire at 2000-01-01 01:00:05.

^Redis does have a single millisecond error rate for expiration (https://redis.io/commands/expire/#expire-accuracy) which means keys may occasionally expire a millisecond after.

[driesvints]

@timacdonald gotcha!