Replace @primer/css-reviewers with @primer/engineer-reviewers in CODEOWNERS (#3996)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jonrohan <54012+jonrohan@users.noreply.github.com>

Author: Copilot

.github/CODEOWNERS

@@ -4,8 +4,8 @@
 .playwright/screenshots/ @primer/design-reviewers
 
 # Component CSS files
-app/components/**/*.pcss @primer/css-reviewers
+app/components/**/*.pcss @primer/engineer-reviewers
 
 # PostCSS Config
-lib/postcss-mixins/ @primer/css-reviewers
-postcss.config.js @primer/css-reviewers
+lib/postcss-mixins/ @primer/engineer-reviewers
+postcss.config.js @primer/engineer-reviewers