Bump postcss and gulp-autoprefixer

Bumps [postcss](https://github.com/postcss/postcss) to 8.4.39 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 8.4.39
- [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/8.4.39)

Updates `gulp-autoprefixer` from 3.1.1 to 9.0.0
- [Release notes](https://github.com/sindresorhus/gulp-autoprefixer/releases)
- [Commits](sindresorhus/gulp-autoprefixer@v3.1.1...v9.0.0)

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

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

package.json

@@ -19,7 +19,7 @@
     "escodegen": "^1.6.1",
     "google-closure-compiler": "",
     "gulp": "^3.9.0",
-    "gulp-autoprefixer": "^3.0.2",
+    "gulp-autoprefixer": "^9.0.0",
     "gulp-cache": "^0.4.5",
     "gulp-closure-compiler": "^0.4.0",
     "gulp-concat": "^2.4.1",