Update webpack requirement from 4.28.3 to 4.28.4

Updates the requirements on [webpack](https://github.com/webpack/webpack) to permit the latest version.
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](https://github.com/webpack/webpack/commits/v4.28.4

)

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