Update dependency purgecss-webpack-plugin to v5

This MR contains the following updates:

Package	Type	Update	Change
purgecss-webpack-plugin (source)	dependencies	major	4.1.3 -> 5.0.0

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Release Notes

FullHuman/purgecss (purgecss-webpack-plugin)

v5.0.0

Compare Source

Bug Fixes

• add safelist variables to postcss-purgecss #​840 (c822058)
• gulp-purgecss: support skippedContentGlobs option #​853 (b72de77)
• Purgecss webpack plugin/only filter fix (#​933) (f8e4c2c)
• purgecss-webpack-plugin: add sourcemap support #​409 (b3f73ed)
• purgecss-webpack-plugin: export as named export as well as default (#​821) (a6a2c8e)
• wrong path import (4f3ddd0)

Features

• add source map support #​526 (f2a9c5a)
• gulp-purgecss: add support for gulp-sourcemaps #​257 (55c26d2)
• postcss-purgecss: load options from purgecss config (4de3bd8)
• purgecss-webpack-plugin: load config file automatically #​767 (726faaa)

BREAKING CHANGES

• purgecss-webpack-plugin: drop webpack 4 support

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