Bump spatie/laravel-sitemap from 5.5.0 to 5.6.0

Bumps [spatie/laravel-sitemap](https://github.com/spatie/laravel-sitemap) from 5.5.0 to 5.6.0.
- [Release notes](https://github.com/spatie/laravel-sitemap/releases)
- [Changelog](https://github.com/spatie/laravel-sitemap/blob/master/CHANGELOG.md)
- [Commits](spatie/laravel-sitemap@5.5.0...5.6.0)

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