Add "Last update" to hover card for entity list

[matthew-white]

Once getodk/central-frontend#1067 is merged, we will still need to add one more piece of information to the hover card for an entity list: "Last update". Right now, the hover card shows "Latest Entity", but we also want it to show "Last update". The value of "Last update" should be similar to what's returned for the ETag: it should be updated whenever an entity in the entity list is created, updated, or deleted. That timestamp isn't returned over the API currently. This issue is to add that timestamp to the API, then add it to the hover card in Frontend.

[matthew-white]

I think I'm going to go ahead and bump this issue to v2025.1. We still need to review and merge getodk/central-frontend#1067, and regression testing is about to begin.