Update darling requirement from 0.20.3 to 0.21.0

Updates the requirements on [darling](https://github.com/TedDriggs/darling) to permit the latest version.
- [Release notes](https://github.com/TedDriggs/darling/releases)
- [Changelog](https://github.com/TedDriggs/darling/blob/master/CHANGELOG.md)
- [Commits](TedDriggs/darling@v0.20.3...v0.20.11)

---
updated-dependencies:
- dependency-name: darling
  dependency-version: 0.20.11
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Cargo.toml

@@ -40,7 +40,7 @@ backon = "1.3"
 base64 = "0.22.1"
 bytes = "1.1.0"
 chrono = { version = "0.4.34", default-features = false }
-darling = "0.20.3"
+darling = "0.21.0"
 derive_more = "2.0.1"
 educe = { version = "0.6.0", default-features = false }
 either = "1.6.1"