Reinstate RedisCacheConfiguration.getTtl() as deprecated

[jxblum]

As of Spring Data Redis 3.2, it is now possible to dynamically compute the TTL of a cache entry (key/value) on expiration.

As such, the static getTtl():Duration method was removed without properly deprecating the method.

This task reinstates the method as deprecated, referring users to use the new getTtlFunction() method instead.

Issue: #2300
Pull Request: #2597

Original Issue: #1433
Original Pull Request: #2587