Update dependency grunt-contrib-cssmin to v4

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
grunt-contrib-cssmin	3.0.0 -> 4.0.0

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Release Notes

gruntjs/grunt-contrib-cssmin

v4.0.0

Compare Source

• Update all dependencies including clean-css v5.x.
• Drop Node.js < 10 support.

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