chore(deps): update darling requirement from 0.14.2 to 0.20.6

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@v0.14.2...v0.20.6)

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

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

sentinel-macros/Cargo.toml

@@ -23,4 +23,4 @@ proc-macro = true
 proc-macro2 = "1.0.33"
 quote = "1.0.9"
 syn = { version = "1.0.74", features = ["full"] }
-darling = "0.14.2"
+darling = "0.20.6"