Update dependency clean-css-cli to v5.6.0

Created by: renovate[bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
clean-css-cli	5.5.2 -> 5.6.0

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Release Notes

clean-css/clean-css-cli

v5.6.0

Compare Source

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• Bumps clean-css dependency to 5.3.0.

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