Bump to Axum 0.8

While working on https://github.com/github/twirp-rs/pull/149 I noticed that Axum has a new major version (0.8) that we have not updated to.

<img width="603" alt="Image" src="https://github.com/user-attachments/assets/0c6391c0-f4ae-4577-b9b0-c2a593163572" />
