Update darling requirement from 0.20 to 0.21 #28

Job	Run time
semver	41s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	5m 5s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 4s
Code Coverage	10s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	3m 7s
Rustfmt	16s
Clippy	3m 55s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	7m 12s
Build Examples (3d_charts)	32s
Build Examples (customization/consistent_static_format_export)	1m 20s
Build Examples (custom_controls)	30s
Build Examples (basic_charts)	32s
Build Examples (customization/density_mapbox_example)	28s
Build Examples (customization/multiple_plots_example)	32s
Build Examples (ndarray)	32s
Build Examples (images)	51s
Build Examples (maps)	41s
Build Examples (kaleido)	1m 19s
Build Examples (shapes)	34s
Build Examples (financial_charts)	30s
Build Examples (statistical_charts)	31s
Build Examples (scientific_charts)	35s
Build Examples (themes)	32s
Build Wasm Examples (basic)	53s
Build Examples (subplots)	28s
Build Book	3m 21s
Build Wasm Examples (callback-example)	54s
	39m 5s

Provide feedback