collection_list.xml: use a <feed> of <entry>s

Instead of having a single <entry> that contains a <deposits> tag
that contains a list of <deposit>

1. Follows the spirit of Atom
2. Follows the AtomPub spec https://datatracker.ietf.org/doc/html/rfc5023\#section-5.2 and the SWORD spec http://swordapp.github.io/SWORDv2-Profile/SWORDProfile.html\#protocoloperations_listingcollections
3. Consistent with the format in state.xml and deposit_receipt.xml
4. Avoids introducing a new tag