Update darling requirement from 0.20 to 0.21

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@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: darling
  dependency-version: 0.21.0
  dependency-type: direct:production
...

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

Author: dependabot[bot]

plotly_derive/Cargo.toml

@@ -14,7 +14,7 @@ keywords = ["plot", "chart", "plotly"]
 quote = "1"
 syn = "2"
 proc-macro2 = "1"
-darling = "0.20"
+darling = "0.21"
 
 [lib]
 proc-macro = true