build(deps): update serial_test requirement from 1 to 2

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@v1.0.0...v2.0.0)

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

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

Author: dependabot[bot]

gio/Cargo.toml

@@ -54,4 +54,4 @@ smallvec = "1"
 futures = "0.3"
 futures-util = { version = "0.3", features = ["io"] }
 gir-format-check = "^0.1"
-serial_test = "1"
+serial_test = "2"