Bump terser-webpack-plugin from 3.0.3 to 3.0.6 (#14238)

Bump terser-webpack-plugin from 3.0.3 to 3.0.6 (#14238)

Bumps [terser-webpack-plugin](https://github.com/webpack-contrib/terser-webpack-plugin) from 3.0.3 to 3.0.6.
- [Release notes](https://github.com/webpack-contrib/terser-webpack-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/terser-webpack-plugin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/terser-webpack-plugin/compare/v3.0.3...v3.0.6)

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