Update worker requirement from 0.2.0 to 0.3.1

Updates the requirements on [worker](https://github.com/cloudflare/workers-rs) to permit the latest version.
- [Release notes](https://github.com/cloudflare/workers-rs/releases)
- [Commits](cloudflare/workers-rs@v0.2.0...v0.3.1)

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

Signed-off-by: dependabot[bot] <support@github.com>

Cargo.toml

@@ -18,7 +18,7 @@ default = ["console_error_panic_hook"]
 cfg-if = "1.0.0"
 # When you change the version of workers, ensure the same version is set for
 # `WORKERS_RS_VERSION` in `wrangler.toml`, especially when dependabot does it
-worker = "0.2.0"
+worker = "0.3.1"
 serde_json = "1.0"
 wasm-bindgen = "^0.2.91"
 wasm-timer = "0.2.5"