feat: upgrade @primer/css from 20.2.4 to 21.0.7

Snyk has created this PR to upgrade @primer/css from 20.2.4 to 21.0.7.

See this package in npm:
https://www.npmjs.com/package/@primer/css

See this project in Snyk:
https://app.snyk.io/org/woodpile37/project/c8124b9c-7460-406a-b6c9-6fac0462b8ac?utm_source=github&utm_medium=referral&page=upgrade-pr

package.json

@@ -12,7 +12,7 @@
     "@elastic/elasticsearch": "7.11.0",
     "@github/failbot": "0.8.2",
     "@primer/behaviors": "^1.3.1",
-    "@primer/css": "^20.2.4",
+    "@primer/css": "^21.0.7",
     "@primer/octicons": "17.10.1",
     "@primer/octicons-react": "17.10.1",
     "@primer/react": "^35.2.2",