Update project to gulp@4

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 https://github.com/gulpjs/gulp/issues/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 https://github.com/gulpjs/gulp/issues/861#issuecomment-874220815 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 [email protected] CHANGELOG.md has a list of breaking changes