Pretty much all the changes are from updating axum to 0.8:

- `async_trait` has been dropped because Return Position `impl Trait`
  (RPIT) makes it unnecessary
- routing placeholders changed from `:foo`/`*foo` to `{foo}`/`{*foo}`
- `Option<extractor>` requires implementing `OptionalFromRequestPart`
- `Option<Query<T>>` doesn't work anymore (tokio-rs/axum#3088) but
  apparently the behaviour was always kinda dodgy
  (tokio-rs/axum#3079)? It's only used for the `/info/refs` service
  query parameter, in which case I can move the `Option` inside the
  struct which cleans up the body (and allows avoiding an allocation)