Bump mini-css-extract-plugin from 0.11.3 to 1.2.0 (#15034)

Bump mini-css-extract-plugin from 0.11.3 to 1.2.0 (#15034)

Bumps [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) from 0.11.3 to 1.2.0.
- [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](https://github.com/webpack-contrib/mini-css-extract-plugin/compare/v0.11.3...v1.2.0)

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