⬆️ Bump terser and laravel-mix in /laravel

Bumps [terser](https://github.com/terser/terser) to 5.15.1 and updates ancestor dependency [laravel-mix](https://github.com/JeffreyWay/laravel-mix). These dependencies need to be updated together.


Updates `terser` from 3.17.0 to 5.15.1
- [Release notes](https://github.com/terser/terser/releases)
- [Changelog](https://github.com/terser/terser/blob/master/CHANGELOG.md)
- [Commits](terser/terser@v3.17.0...v5.15.1)

Updates `laravel-mix` from 5.0.4 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: terser
  dependency-type: indirect
- dependency-name: laravel-mix
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>