Bump webpack from 4.20.2 to 4.21.0

Bumps [webpack](https://github.com/webpack/webpack) from 4.20.2 to 4.21.0.
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](https://github.com/webpack/webpack/compare/v4.20.2...v4.21.0

)

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