Update maxmemory policy descriptions

[Berdir]

Why

The valkey (and redis) documentation talks about an expire field being true/false. There is no such thing. I suspect this got mixed up with the Drupal redis module, which stores a such a flag as part of it's cache items.

What valkey/redis does have is a TTL, set with EXPIRE and read with TTL.

What's changed

Copy paste of the descriptions from the valkey docs which consistently use TTL instead of expire

Where are changes

Updates are for:

• [x] platform (sites/platform templates)
• [ ] upsun (sites/upsun templates)