Update dependency gulp-autoprefixer to v8

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
gulp-autoprefixer	7.0.0 -> 8.0.0

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Release Notes

sindresorhus/gulp-autoprefixer

v8.0.0

Compare Source

Breaking

• Require Node.js 12 a953087

Improvements

• Upgrade dependencies a953087

v7.0.1

Compare Source

• Make Gulp an optional peer dependency da50fd2

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