Update darling requirement from 0.20 to 0.21 #27

Job	Run time
Build Wasm Examples (basic)	50s
Clippy	4m 0s
semver	51s
Rustfmt	16s
Code Coverage	10s
Tests (windows-latest, chrome, plotly_ndarray,plotly_image,static_export_chromedriver,debug)	7m 14s
Build Book	3m 24s
Tests (macos-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 9s
Build Examples (custom_controls)	33s
Build Examples (customization/consistent_static_format_export)	1m 23s
Build Examples (basic_charts)	34s
Build Wasm Examples (callback-example)	55s
Tests (ubuntu-latest, firefox, plotly_ndarray,plotly_image,static_export_geckodriver,static_expor...	3m 2s
Build Examples (customization/multiple_plots_example)	31s
Build Examples (ndarray)	31s
Build Examples (financial_charts)	26s
Build Examples (kaleido)	1m 29s
Build Examples (maps)	42s
Build Examples (3d_charts)	30s
Build Examples (images)	43s
Build Examples (themes)	30s
Build Examples (scientific_charts)	35s
Build Examples (subplots)	29s
Build Examples (statistical_charts)	34s
Build Examples (shapes)	38s
Tests (ubuntu-latest, chrome, plotly_ndarray,plotly_image,static_export_default,debug)	3m 6s
Build Examples (customization/density_mapbox_example)	27s
	37m 32s

Provide feedback