This adds support for `ghc-9.12` / `hoauth2-2.15` and drops support for
`ghc < 9.4` / `hoauth2 < 2.8`.
Since this would be a major version bump no matter what, I've changed
the interface we present to align with `hoauth2-2.15`. This means using
the newer `fetch` functions, and `TokenResponse{,Error}` type names.
I've maintained our own `OAuth2` type so that the redirect-uri can
remain a `Maybe` field. The way plugins are constructed, we need to
build an `OAuth2` value in a pure context without one, which is then
supplied later, when we have `MonadHandler` and so can render URLs.
