chore(deps): update dependency cssnano to v8 (#507)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package.json

@@ -92,7 +92,7 @@
         "@tryghost/theme-translations": "^0.0.9",
         "autoprefixer": "10.5.0",
         "beeper": "2.1.0",
-        "cssnano": "7.1.9",
+        "cssnano": "8.0.1",
         "gscan": "5.4.3",
         "gulp": "5.0.1",
         "gulp-concat": "2.6.1",