Bump tough-cookie and gulp-sass in /assets

Removes [tough-cookie](https://github.com/salesforce/tough-cookie). It's no longer used after updating ancestor dependency [gulp-sass](https://github.com/dlmanning/gulp-sass). These dependencies need to be updated together.


Removes `tough-cookie`

Updates `gulp-sass` from 4.0.1 to 5.1.0
- [Release notes](https://github.com/dlmanning/gulp-sass/releases)
- [Changelog](https://github.com/dlmanning/gulp-sass/blob/master/CHANGELOG.md)
- [Commits](dlmanning/gulp-sass@v4.0.1...v5.1.0)

---
updated-dependencies:
- dependency-name: tough-cookie
  dependency-type: indirect
- dependency-name: gulp-sass
  dependency-type: direct:development
...

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