Use "URL" instead of "URI" everywhere when talking about database URLs

[abonander]

The use of the term "URI" is discouraged by the WHATWG: https://url.spec.whatwg.org/#goals

Standardize on the term URL. URI and IRI are just confusing. In practice a single algorithm is used for both so keeping them distinct is not helping anyone. URL also easily wins the search result popularity contest.

We already have DATABASE_URL and that's not going anywhere, so using "URI" in other places to refer to the same string format is just unnecessarily confusing.

This should be more or less a trivial find-replace so if someone wants an easy blip on their "Contributions" graph, this is a good issue to tackle. I'm only classifying this E-medium if someone wants to cover the whole repo at once.