Bump stylelint from 15.6.0 to 16.15.0 #127

Job	Run time
Build (github-pages gem)	44s
Build (jekyll gem) (3.9, ubuntu-latest, 2.7)	1m 3s
Test CSS and JS (18.x)	6s
Build (jekyll gem) (3.9, ubuntu-latest, 3.1)	24s
Build (jekyll gem) (3.9, macos-latest, 2.7)	1m 3s
Build (jekyll gem) (3.9, macos-latest, 3.1)	28s
Build (jekyll gem) (3.9, windows-latest, 2.7)	5m 22s
Build (jekyll gem) (3.9, windows-latest, 3.1)	2m 19s
Build (jekyll gem) (4.3, ubuntu-latest, 2.7)	1m 32s
Build (jekyll gem) (4.3, ubuntu-latest, 3.1)	36s
Build (jekyll gem) (4.3, macos-latest, 2.7)	1m 16s
Build (jekyll gem) (4.3, macos-latest, 3.1)	33s
Build (jekyll gem) (4.3, windows-latest, 2.7)	3m 47s
Build (jekyll gem) (4.3, windows-latest, 3.1)	2m 51s
	22m 4s

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