Update quick-xml requirement from 0.20.0 to 0.21.0
Update quick-xml requirement from 0.20.0 to 0.21.0

Updates the requirements on [quick-xml](https://github.com/tafia/quick-xml) to permit the latest version.
- [Release notes](https://github.com/tafia/quick-xml/releases)
- [Changelog](https://github.com/tafia/quick-xml/blob/master/Changelog.md)
- [Commits](https://github.com/tafia/quick-xml/compare/v0.20.0...v0.21.0)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
CryZe/livesplit-core,CryZe/livesplit-core