build(deps): update rstest requirement from 0.25 to 0.26

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@v0.25.0...v0.26.1)

---
updated-dependencies:
- dependency-name: rstest
  dependency-version: 0.26.1
  dependency-type: direct:production
...

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

Author: dependabot[bot]

cdevents-sdk/Cargo.toml

@@ -26,7 +26,7 @@ boon = "0.6"
 glob = "0.3"
 proptest = "1"
 regex = "1.10"
-rstest = "0.25"
+rstest = "0.26"
 
 [features]
 default = ["cloudevents"]