Bump autoprefixer from 10.4.20 to 10.4.21 (#1113)

Bumps [autoprefixer](https://github.com/postcss/autoprefixer) from 10.4.20 to 10.4.21.
- [Release notes](https://github.com/postcss/autoprefixer/releases)
- [Changelog](https://github.com/postcss/autoprefixer/blob/main/CHANGELOG.md)
- [Commits](postcss/autoprefixer@10.4.20...10.4.21)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

package.json

@@ -17,7 +17,7 @@
     "url": "https://github.com/textpattern/textpattern-jquery-ui-theme/issues"
   },
   "devDependencies": {
-    "autoprefixer": "10.4.20",
+    "autoprefixer": "10.4.21",
     "clean-webpack-plugin": "4.0.0",
     "cross-env": "7.0.3",
     "css-loader": "7.1.2",