fix: udeps build

[chris13524]

udeps action no longer runs with axum 0.7.5 crate due to outdated nightly version and attempting to use feature from newer nightly. Bad idea to lock on an old nightly version. Looks like this PR fixed the root cause as to why we had to lock nightly versions.

https://github.com/WalletConnect/notify-server/actions/runs/8560316766/job/23459012382?pr=467

error[E0658]: `#[diagnostic]` attribute name space is experimental
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/axum-0.7.5/src/handler/mod.rs:130:5
    |
130 |     diagnostic::on_unimplemented(
    |     ^^^^^^^^^^
    |
    = note: see issue #111996 <https://github.com/rust-lang/rust/issues/111996> for more information
    = help: add `#![feature(diagnostic_namespace)]` to the crate attributes to enable
    = note: this compiler was built on 2024-02-03; consider upgrading it if it is out of date

Also switch cargo fmt version from nightly to stable. Not sure why this was on nightly; we are supposed to be using stable for everything.