build(deps-dev): bump mini-css-extract-plugin in /frontend

Bumps [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) from 1.3.1 to 1.3.4.
- [Release notes](https://github.com/webpack-contrib/mini-css-extract-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/mini-css-extract-plugin/blob/master/CHANGELOG.md)
- [Commits](webpack-contrib/mini-css-extract-plugin@v1.3.1...v1.3.4)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]