Update dependency purgecss-webpack-plugin to v6

This MR contains the following updates:

Package	Type	Update	Change
purgecss-webpack-plugin (source)	dependencies	major	5.0.0 -> 6.0.0

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Release Notes

FullHuman/purgecss (purgecss-webpack-plugin)

v6.0.0

Compare Source

Bug Fixes

• collect of used variables for changed selector (#​1217) (9fbb592)
• gulp-purgecss: move vinyl-sourcemaps-apply to deps #​999 (7858b7a)

Features

• purgecss: add support for :where and :is #​978 (bb5782b)
• warn if no files are found #​642 (6188505)

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