Bump @primer/css from 14.4.0 to 15.1.0

Bumps [@primer/css](https://github.com/primer/css) from 14.4.0 to 15.1.0.
- [Release notes](https://github.com/primer/css/releases)
- [Changelog](https://github.com/primer/css/blob/master/CHANGELOG.md)
- [Commits](primer/css@v14.4.0...v15.1.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -6,7 +6,7 @@
  "license": "MIT",
  "bugs": "https://github.com/pmarsceill/just-the-docs/issues",
  "devDependencies": {
- "@primer/css": "^14.4.0",
+ "@primer/css": "^15.1.0",
  "prettier": "^2.0.5",
  "stylelint": "^13.6.1",
  "stylelint-config-prettier": "^8.0.2",