Bump laravel/framework from 6.20.15 to 6.20.19 (#550)

Bumps [laravel/framework](https://github.com/laravel/framework) from 6.20.15 to 6.20.19.
- [Release notes](https://github.com/laravel/framework/releases)
- [Changelog](https://github.com/laravel/framework/blob/8.x/CHANGELOG-6.x.md)
- [Commits](laravel/framework@v6.20.15...v6.20.19)

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