Bump axum to 0.8 in example

[rikhuijzer]

This PR updates the Axum version to the latest in the example.

I ran this example locally to verify that it works and I also have updated another project of mine to axum 0.8 and it works in production.

Currently, it does show an error

warning: this function call involves an argument of type `()` which is affected by the wasm ABI transition
 --> /Users/rik/git/workers-rs/worker-sys/src/types/rate_limit.rs:4:1
  |
4 | #[wasm_bindgen::prelude::wasm_bindgen]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #138762 <https://github.com/rust-lang/rust/issues/138762>
  = help: the "C" ABI Rust uses on wasm32-unknown-unknown will change to align with the standard "C" ABI for this target
  = note: this warning originates in the attribute macro `wasm_bindgen::prelude::wasm_bindgen` (in Nightly builds, run with -Z macro-backtrace for more info)

But this is a false positive according to rustwasm/wasm-bindgen#3454 (comment). It should be fixed in newer Rust versions according to rustwasm/wasm-bindgen#3454 (comment)