Update terser-webpack-plugin requirement from 1.1.0 to 1.2.1 (#3867)

Updates the requirements on [terser-webpack-plugin](https://github.com/webpack-contrib/terser-webpack-plugin) to permit the latest version.
- [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/commits/v1.2.1

)

Signed-off-by: dependabot[bot] <support@dependabot.com>