Cache clean for items older than x

[dtrudg]

#5088 disables the code to remove a specific cache item.

This will be returned in a future refactor that moves the body of the cache deletion code into the cache package, out of the CLI.

[dtrudg]

Changing this issue to reflect that after discussion it has been decided to implement cache clean of items older than a specified duration. The new cache format doesn't save the names of any items - only their hashes which are not really useful to clean specifically. This was also mostly the case previously as blob and oci-tmp caches used hashes for entry names.

The though of doing it by age instead has also come up in discussion on slack more than once. It seems generally more useful.