Update project to gulp@4

[Splaktar]

Discuss the use-case or motivation for changing the existing behavior

We need to support using the non-deprecated sass package instead of only the deprecated node-sass package. But to do this, we need to update to gulp-sass@5 which requires gulp@4.

List the affected versions of AngularJS, Material, OS, and browsers

• AngularJS: 1.2.2
• AngularJS Material: 1.8.2
• OS: macOS
• Browsers: Chrome
• Screen Readers: VoiceOver

Add anything else we should know

• In [Docs] Migration Guide gulpjs/gulp#861, there is some discussion of a migration guide, but there does not actually seem to be a comprehensive guide, just some mentions of things scattered throughout the docs like
  • series()
  • parallel()
• I also mentioned in [Docs] Migration Guide gulpjs/gulp#861 (comment) that Releases page for the v4.0.0-alpha.x releases has links to the commits, but the commits don't have comments or migration instructions
• The gulp@4.0.0 CHANGELOG.md has a list of breaking changes

[Splaktar]

Based on PR #12131, this doesn't seem to be blocking gulp-sass@5 afterall.