Bump form-data and laravel-mix (#458)

Removes [form-data](https://github.com/form-data/form-data). It's no longer used after updating ancestor dependency [laravel-mix](https://github.com/JeffreyWay/laravel-mix). These dependencies need to be updated together.


Removes `form-data`

Updates `laravel-mix` from 2.1.1 to 6.0.49
- [Release notes](https://github.com/JeffreyWay/laravel-mix/releases)
- [Changelog](https://github.com/laravel-mix/laravel-mix/blob/master/CHANGELOG.md)
- [Upgrade guide](https://github.com/laravel-mix/laravel-mix/blob/master/UPGRADE.md)
- [Commits](https://github.com/JeffreyWay/laravel-mix/commits)

---
updated-dependencies:
- dependency-name: form-data
  dependency-version: 
  dependency-type: indirect
- dependency-name: laravel-mix
  dependency-version: 6.0.49
  dependency-type: direct:development
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Manel Gavalda <manelgavalda1@gmail.com>

Author: dependabot[bot]