Bump gulp-autoprefixer from 8.0.0 to 9.0.0

Bumps [gulp-autoprefixer](https://github.com/sindresorhus/gulp-autoprefixer) from 8.0.0 to 9.0.0.
- [Release notes](https://github.com/sindresorhus/gulp-autoprefixer/releases)
- [Commits](sindresorhus/gulp-autoprefixer@v8.0.0...v9.0.0)

---
updated-dependencies:
- dependency-name: gulp-autoprefixer
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

package.json

@@ -24,7 +24,7 @@
   "homepage": "git+https://github.com/lightspeeddevelopment/lsx-customizer",
   "devDependencies": {
     "gulp": "4.0.2",
-    "gulp-autoprefixer": "8.0.0",
+    "gulp-autoprefixer": "9.0.0",
     "gulp-concat": "2.6.1",
     "gulp-gettext": "0.3.0",
     "gulp-jshint": "2.1.0",