Update dependency clean-css to v4.2.4

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
clean-css	4.2.1 -> 4.2.4

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Release Notes

clean-css/clean-css (clean-css)

### [`v4.2.4`](https://togithub.com/clean-css/clean-css/compare/v4.2.3...v4.2.4) [Compare Source](https://togithub.com/clean-css/clean-css/compare/v4.2.3...v4.2.4) ### [`v4.2.3`](https://togithub.com/clean-css/clean-css/blob/HEAD/History.md#423--2020-01-28) [Compare Source](https://togithub.com/clean-css/clean-css/compare/v4.2.2...v4.2.3) \================== - Fixed issue [#​1106](https://togithub.com/clean-css/clean-css/issues/1106) - regression in handling RGBA/HSLA colors. ### [`v4.2.2`](https://togithub.com/clean-css/clean-css/blob/HEAD/History.md#422--2020-01-25) [Compare Source](https://togithub.com/clean-css/clean-css/compare/v4.2.1...v4.2.2) \================== - Fixed error when property block has no value. - Fixed issue [#​1077](https://togithub.com/clean-css/clean-css/issues/1077) - local fonts with color in name. - Fixed issue [#​1082](https://togithub.com/clean-css/clean-css/issues/1082) - correctly convert colors if alpha >= 1. - Fixed issue [#​1085](https://togithub.com/clean-css/clean-css/issues/1085) - prevent unquoting of grid elements.

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Configuration

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