Bump terser, laravel-mix and laravel-mix-purgecss

Bumps [terser](https://github.com/terser/terser) to 5.16.6 and updates ancestor dependencies [terser](https://github.com/terser/terser), [laravel-mix](https://github.com/JeffreyWay/laravel-mix) and [laravel-mix-purgecss](https://github.com/spatie/laravel-mix-purgecss). These dependencies need to be updated together.


Updates `terser` from 3.17.0 to 5.16.6
- [Release notes](https://github.com/terser/terser/releases)
- [Changelog](https://github.com/terser/terser/blob/master/CHANGELOG.md)
- [Commits](terser/terser@v3.17.0...v5.16.6)

Updates `laravel-mix` from 4.1.4 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)

Updates `laravel-mix-purgecss` from 4.2.0 to 6.0.0
- [Release notes](https://github.com/spatie/laravel-mix-purgecss/releases)
- [Changelog](https://github.com/spatie/laravel-mix-purgecss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/spatie/laravel-mix-purgecss/commits/6.0.0)

---
updated-dependencies:
- dependency-name: terser
  dependency-type: indirect
- dependency-name: laravel-mix
  dependency-type: direct:development
- dependency-name: laravel-mix-purgecss
  dependency-type: direct:development
...

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

Author: dependabot[bot]