update darling requirement from ^0.14 to ^0.20

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@v0.14.0...v0.20.3)

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

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

robusta-codegen/Cargo.toml

@@ -18,5 +18,5 @@ proc-macro2 = { version = "1.0.21", features = ["span-locations"]}
 syn = { version = "^1", features = ["visit", "fold", "derive"] }
 proc-macro-error = { version = "1", default-features = false }
 rand = "^0"
-darling = "^0.14"
+darling = "^0.20"
 Inflector = "0.11.4"