Update cache TTL interval to match HTTP cache control headers

[momo-ozawa]

https://github.com/wordpress-mobile/WordPress-iOS/pull/21348#discussion_r1303657616

It's probably not ideal to cache avatars in memory without providing a time-to-live interval that roughly matches the HTTP cache control headers (around 5 minutes?). If you change the outside of the app, it won't update until the memory cache clears, which may need to happen while the app is running.