chore(deps): update dependency gulp-autoprefixer to v7

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
gulp-autoprefixer	devDependencies	major	6.1.0 -> 7.0.0

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Release Notes

sindresorhus/gulp-autoprefixer

### [`v7.0.0`](https://togithub.com/sindresorhus/gulp-autoprefixer/releases/v7.0.0) [Compare Source](https://togithub.com/sindresorhus/gulp-autoprefixer/compare/v6.1.0...v7.0.0) Breaking: - Require Node.js 8 and Gulp 4 [`ab49120`](https://togithub.com/sindresorhus/gulp-autoprefixer/commit/ab49120)

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