bump axum-core

[NicolaLS]

running cargo audit I got this:
Crate: axum-core
Version: 0.1.2
Title: No default limit put on request bodies
Date: 2022-08-31
ID: RUSTSEC-2022-0055
URL: https://rustsec.org/advisories/RUSTSEC-2022-0055
Solution: Upgrade to >=0.2.8, <0.3.0-rc.1 OR >=0.3.0-rc.2
Dependency tree:

[djc]

I don't think there is a need to bump this dependency. All crates that update Askama will likely also update axum-core and get 0.2.8 as part of that update.