build(deps-dev): bump mini-css-extract-plugin from 1.6.0 to 2.1.0 (#729)

Bumps [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) from 1.6.0 to 2.1.0.
- [Release notes](https://github.com/webpack-contrib/mini-css-extract-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/mini-css-extract-plugin/blob/v2.1.0/CHANGELOG.md)
- [Commits](webpack/mini-css-extract-plugin@v1.6.0...v2.1.0)

---
updated-dependencies:
- dependency-name: mini-css-extract-plugin
  dependency-type: direct:development
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

packages/purgecss-webpack-plugin/package-lock.json

@@ -46,7 +46,7 @@
   "devDependencies": {
     "@types/webpack-sources": "^2.0.0",
     "css-loader": "^5.0.0",
-    "mini-css-extract-plugin": "^1.2.0"
+    "mini-css-extract-plugin": "^2.1.0"
   },
   "peerDependencies": {
     "webpack": "*"