[Feature] Add /wp-content/cache to default exclusion rules

[joe9663]

Is your feature request related to a problem? Please describe.
Some websites encounter issues generating backups when /wp-content/cache is included in the backup archive. This problem was witnessed specifically with W3TC implemented, but it is likely that other caching plugins could create similar issues.

[jessecowens]

+1 from customer feedback in the internal slack