build(deps): update rstest requirement from 0.18 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@v0.18.0...v0.21.0)

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

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

Author: dependabot[bot]

cdevents-sdk/Cargo.toml

@@ -24,7 +24,7 @@ assert-json-diff = "2.0"
 boon = "0.5"
 glob = "0.3"
 proptest = "1"
-rstest = "0.18"
+rstest = "0.21"
 
 [features]
 default = ["cloudevents"]