Update serial_test requirement from 2.0.0 to 3.0.0 (#280)

Updates the requirements on [serial_test](https://github.com/palfrey/serial_test) to permit the latest version.
- [Release notes](https://github.com/palfrey/serial_test/releases)
- [Commits](palfrey/serial_test@v2.0.0...v3.0.0)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

vergen/Cargo.toml

@@ -61,7 +61,7 @@ gix = { version = "0.57.1", default-features = false, features = [
 ] }
 lazy_static = "1.4.0"
 regex = "1.9.1"
-serial_test = "2.0.0"
+serial_test = "3.0.0"
 
 [package.metadata.docs.rs]
 features = ["build", "cargo", "git", "gitcl", "rustc", "si"]