[ios] Move cache.db to a subdirectory (#5601)

We cannot guarantee that the offline/ambient tile cache (cache.db) will be created in a timely manner, so it's safer to set the backup exclusion key on its containing directory.

But, because we placed cache.db in the base of the app's Application Support directory (where backups are expected to be allowed), we need to move cache.db into a directory that we control and can exclude from backups with impunity.

This is an amalgam of https://github.com/mapbox/mapbox-gl-native/pull/5578 (directory exclusion) and https://github.com/mapbox/mapbox-gl-native/pull/5585 (refactored URL methods, but without the new database size method).

Also:
* Prefix mapbox subdirectory with dot, for extra safety
* Update changelog entry with new ticket, less speculation

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