[Snyk] Upgrade @primer/css from 15.1.0 to 21.3.6#6

InfiniteLove2020 wants to merge 1 commit intomainInfiniteLove2020/docs-2:mainfrom

snyk-upgrade-df80987449a8dbfd2a369b3c04ea7191InfiniteLove2020/docs-2:snyk-upgrade-df80987449a8dbfd2a369b3c04ea7191Copy head branch name to clipboard