Update serial_test requirement from 2.0 to 3.1

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...v3.1.1)

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

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

zirgen/dsl/Cargo.toml

@@ -15,7 +15,7 @@ pretty_assertions = "1.4"
 rayon = "1.8"
 risc0-core = { workspace = true }
 risc0-zkp = { workspace = true, default-features = false, features = ["prove"] }
-serial_test = "2.0"
+serial_test = "3.1"
 strum = { version = "0.26", features = ["derive"] }
 strum_macros = "0.26"
 test-log = { version = "0.2", features = ["trace"] }