Update dependency gulp-autoprefixer to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change	References
gulp-autoprefixer	devDependencies	major	3.1.1 -> 6.0.0	source

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Release Notes

sindresorhus/gulp-autoprefixer

v6.0.0

Compare Source

v5.0.0

Compare Source

v4.1.0

Compare Source

v4.0.0

Compare Source

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[coveralls]

Coverage remained the same at 81.25% when pulling 6fefe1d on renovate/gulp-autoprefixer-6.x into e1e4b5f on master.

[coveralls]

Coverage remained the same at 81.25% when pulling 6fefe1d on renovate/gulp-autoprefixer-6.x into e1e4b5f on master.

[coveralls]

Coverage remained the same at 81.25% when pulling 6fefe1d on renovate/gulp-autoprefixer-6.x into e1e4b5f on master.

[renovate]

PR has been edited

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