Upgrade: Bump postcss-loader from 5.3.0 to 6.1.0

Bumps [postcss-loader](https://github.com/webpack-contrib/postcss-loader) from 5.3.0 to 6.1.0.
- [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/v5.3.0...v6.1.0)

---
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

@@ -12,7 +12,7 @@
     "@vuepress/plugin-back-to-top": "^2.0.0-beta.16",
     "@vuepress/plugin-debug": "^2.0.0-beta.15",
     "@vuepress/plugin-search": "^2.0.0-beta.16",
-    "postcss-loader": "^5.3.0",
+    "postcss-loader": "^6.1.0",
     "prettier": "^2.3.0",
     "sass": "1.32.13",
     "sass-loader": "^12.0.0",

yarn.lock

@@ -3323,6 +3323,15 @@ postcss-loader@^5.3.0:
     klona "^2.0.4"
     semver "^7.3.4"
 
+postcss-loader@^6.1.0:
+  version "6.1.0"
+  resolved "https://registry.yarnpkg.com/postcss-loader/-/postcss-loader-6.1.0.tgz#13c800a00b3c7d434a1319a73d85e2df0714cdde"
+  integrity sha512-yA/cXBfACkthZNA2hQxOnaReVfQ6uLmvbEDQzNafpbK40URZJvP/28dL1DG174Gvz3ptkkHbbwDBCh+gXR94CA==
+  dependencies:
+    cosmiconfig "^7.0.0"
+    klona "^2.0.4"
+    semver "^7.3.5"
+
 postcss-modules-extract-imports@^3.0.0:
   version "3.0.0"
   resolved "https://registry.yarnpkg.com/postcss-modules-extract-imports/-/postcss-modules-extract-imports-3.0.0.tgz#cda1f047c0ae80c97dbe28c3e76a43b88025741d"