Bump postcss and gulp-autoprefixer in /zurichBook

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.21 and updates ancestor dependency [gulp-autoprefixer](https://github.com/sindresorhus/gulp-autoprefixer). These dependencies need to be updated together.


Updates `postcss` from 5.2.18 to 8.4.21
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](https://github.com/postcss/postcss/commits/8.4.21)

Updates `gulp-autoprefixer` from 3.1.1 to 8.0.0
- [Release notes](https://github.com/sindresorhus/gulp-autoprefixer/releases)
- [Commits](sindresorhus/gulp-autoprefixer@v3.1.1...v8.0.0)

---
updated-dependencies:
- dependency-name: postcss
  dependency-type: indirect
- dependency-name: gulp-autoprefixer
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

zurichBook/package.json

@@ -11,7 +11,7 @@
   "devDependencies": {
     "browser-sync": "^2.18.13",
     "gulp": "^3.9.1",
-    "gulp-autoprefixer": "^3.1.1",
+    "gulp-autoprefixer": "^8.0.0",
     "gulp-cssbeautify": "^0.1.3",
     "gulp-sass": "^3.1.0"
   }