Bump postcss

Bumps the npm_and_yarn group with 1 update in the /themes/default directory: [postcss](https://github.com/postcss/postcss).


Updates `postcss` from 7.0.39 to 8.5.6
- [Release notes](https://github.com/postcss/postcss/releases)
- [Changelog](https://github.com/postcss/postcss/blob/main/CHANGELOG.md)
- [Commits](postcss/postcss@7.0.39...8.5.6)

---
updated-dependencies:
- dependency-name: postcss
  dependency-version: 8.5.6
  dependency-type: indirect
  dependency-group: npm_and_yarn
...

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

Author: dependabot[bot]

themes/default/package-lock.json

@@ -16,7 +16,7 @@
     "url": "https://github.com/omeka-s-themes/default/issues"
   },
   "devDependencies": {
-    "autoprefixer": "^9.2.1",
+    "autoprefixer": "^10.4.24",
     "gulp": "^5.0.0",
     "gulp-clean-css": "^4.3.0",
     "gulp-concat": "^2.6.1",