Fix security vulnerabilities (Gulp v4, cssnano, gulp-mocha v7 upgrades)

[hannalaakso]

There are currently multiple security vulnerabilities reported for npm packages:

found 12 vulnerabilities (3 low, 1 moderate, 7 high, 1 critical) in 10514 scanned packages
9 vulnerabilities require semver-major dependency updates.
3 vulnerabilities require manual review. See the full report for details.

To resolve the alerts, this PR:

• Updates from Gulp 3.91 to Gulp 4.0.2 (latest)
• Updates project Gulp tasks to use Gulp 4 (with some minor tidy up)
• Replaces deprecated gulp-cssnano with cssnano via gulp-postcss
• Updates gulp-mocha three major versions up (4.3.1 -> 7.0.2)

Recommend reviewing the commits individually.

How has this been tested?

• npm test passes and fails automated tests appropriately
• npm start removes the /public folder, outputs both a minified and non-minified version of CSS into /public/stylesheets, copies JavaScript to /public/javascripts, copies images to /public/images and starts the nodemon server
• 'cssnano' with 'gulp-postcsss' (which replace deprecated 'gulp-cssnano') minify /public/stylesheets/govuk-elements-styles.min.css file correctly
• gulp package generates /package/ with changes for publishing
• gulp default outputs the available main tasks correctly

What type of change is it?

• Fixing security vulnerabilities

Has the documentation been updated?

• My change requires no change to the documentation.
• I have read the CONTRIBUTING document.

Fixes https://github.com/alphagov/design-system-team-internal/issues/285