chore(deps) Update dependency gulp-purgecss to v6

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
gulp-purgecss (source)	5.0.0 -> 6.0.0

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Release Notes

FullHuman/purgecss (gulp-purgecss)

### [`v6.0.0`](https://togithub.com/FullHuman/purgecss/blob/HEAD/CHANGELOG.md#600-2024-03-29) [Compare Source](https://togithub.com/FullHuman/purgecss/compare/v5.0.0...v6.0.0) ##### Bug Fixes - collect of used variables for changed selector ([#​1217](https://togithub.com/FullHuman/purgecss/issues/1217)) ([9fbb592](https://togithub.com/FullHuman/purgecss/commit/9fbb5920164d3ffd296c0eaab6a4d2b4477b418c)) - **gulp-purgecss:** move vinyl-sourcemaps-apply to deps [#​999](https://togithub.com/FullHuman/purgecss/issues/999) ([7858b7a](https://togithub.com/FullHuman/purgecss/commit/7858b7a910efda13162095d8f528fb9e668cc190)) ##### Features - **purgecss:** add support for :where and :is [#​978](https://togithub.com/FullHuman/purgecss/issues/978) ([bb5782b](https://togithub.com/FullHuman/purgecss/commit/bb5782bff296805f95c6bae0ce434be314bd1580)) - warn if no files are found [#​642](https://togithub.com/FullHuman/purgecss/issues/642) ([6188505](https://togithub.com/FullHuman/purgecss/commit/61885055f46bc36c839f77c07e7d680378083567))

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Configuration

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