Bump serenity from 0.12.2 to 0.12.4 in /templates/rust

Bumps [serenity](https://github.com/serenity-rs/serenity) from 0.12.2 to 0.12.4.
- [Release notes](https://github.com/serenity-rs/serenity/releases)
- [Changelog](https://github.com/serenity-rs/serenity/blob/current/CHANGELOG.md)
- [Commits](serenity-rs/serenity@v0.12.2...v0.12.4)

---
updated-dependencies:
- dependency-name: serenity
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

templates/rust/Cargo.toml

@@ -6,5 +6,5 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-serenity = { version = "0.12.2", default-features = false, features = ["client", "gateway", "rustls_backend", "model"] }
+serenity = { version = "0.12.4", default-features = false, features = ["client", "gateway", "rustls_backend", "model"] }
 tokio = { version = "1.39.2", features = ["macros", "rt-multi-thread"] }