Upgrade: Bump postcss-loader from 6.2.1 to 7.0.0

Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 6.2.1 to 7.0.0.
- [Release notes](https://github.com/webpack-contrib/postcss-loader/releases)
- [Changelog](https://github.com/webpack-contrib/postcss-loader/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/postcss-loader/compare/v6.2.1...v7.0.0)

---
updated-dependencies:
- dependency-name: postcss-loader
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

package.json

@@ -14,7 +14,7 @@
     "@vuepress/plugin-back-to-top": "^2.0.0-beta.25",
     "@vuepress/plugin-debug": "^2.0.0-beta.25",
     "@vuepress/plugin-search": "^2.0.0-beta.25",
-    "postcss-loader": "^6.2.1",
+    "postcss-loader": "^7.0.0",
     "prettier": "^2.6.2",
     "sass": "^1.51.0",
     "sass-loader": "^13.0.0",