Update rstest requirement from 0.19 to 0.21

Updates the requirements on [rstest](https://github.com/la10736/rstest) to permit the latest version.
- [Release notes](https://github.com/la10736/rstest/releases)
- [Changelog](https://github.com/la10736/rstest/blob/master/CHANGELOG.md)
- [Commits](la10736/rstest@v0.19.0...v0.21.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

thirtyfour/Cargo.toml

@@ -73,7 +73,7 @@ reqwest = { version = "0.12", default-features = false, features = [
 assert_matches = "1.5"
 axum = "0.7"
 color-eyre = "0.6"
-rstest = "0.19"
+rstest = "0.21"
 tower-http = { version = "0.5", features = ["fs"] }
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }