Update axum requirement from 0.7 to 0.8

Updates the requirements on [axum](https://github.com/tokio-rs/axum) to permit the latest version.
- [Release notes](https://github.com/tokio-rs/axum/releases)
- [Changelog](https://github.com/tokio-rs/axum/blob/main/CHANGELOG.md)
- [Commits](https://github.com/tokio-rs/axum/commits)

---
updated-dependencies:
- dependency-name: axum
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

askama_axum/Cargo.toml

@@ -19,7 +19,7 @@ axum-core = "0.4"
 http = "1.0"
 
 [dev-dependencies]
-axum = { version = "0.7", default-features = false }
+axum = { version = "0.8", default-features = false }
 http-body-util = "0.1"
 tokio = { version = "1.0", features = ["macros", "rt"] }
 tower = "0.5"