fix: upgrade @primer/css from 21.3.1 to 21.3.6

Snyk has created this PR to upgrade @primer/css from 21.3.1 to 21.3.6.

See this package in npm:
@primer/css

See this project in Snyk:
https://app.snyk.io/org/playgirlkaybraz11-KTz7vPn4RY3tSmDzVJLKFu/project/98d5bc36-ad61-43f4-9f27-782d1eefbf3c?utm_source=github-cloud-app&utm_medium=referral&page=upgrade-pr

package.json

@@ -200,7 +200,7 @@
     "@octokit/plugin-retry": "6.0.1",
     "@octokit/request-error": "6.1.1",
     "@primer/behaviors": "^1.5.1",
-    "@primer/css": "^21.3.1",
+    "@primer/css": "^21.3.6",
     "@primer/octicons": "^19.9.0",
     "@primer/octicons-react": "^19.9.0",
     "@primer/react": "36.19.0",