Bump stylelint from 15.6.0 to 16.16.0 #129

Job	Run time
Build (github-pages gem)	42s
Test CSS and JS (18.x)	7s
Build (jekyll gem) (3.9, macos-latest, 3.1)	24s
Build (jekyll gem) (3.9, ubuntu-latest, 3.1)	25s
Build (jekyll gem) (3.9, macos-latest, 2.7)	59s
Build (jekyll gem) (3.9, ubuntu-latest, 2.7)	1m 18s
Build (jekyll gem) (3.9, windows-latest, 3.1)	2m 13s
Build (jekyll gem) (4.3, windows-latest, 2.7)	4m 25s
Build (jekyll gem) (4.3, ubuntu-latest, 3.1)	33s
Build (jekyll gem) (4.3, windows-latest, 3.1)	2m 21s
Build (jekyll gem) (4.3, ubuntu-latest, 2.7)	1m 28s
Build (jekyll gem) (4.3, macos-latest, 3.1)	31s
Build (jekyll gem) (3.9, windows-latest, 2.7)	3m 46s
Build (jekyll gem) (4.3, macos-latest, 2.7)	1m 13s
	20m 25s

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