Add imagemin to the list of Grunt dependencies.

[desrosj]

The matchdep package was removed in r59797 in an effort to remove dependencies that are no longer maintained.

It appears that the grunt-contrib-imagemin package was missed when creating the gruntDependencies list to replace the functionality of matchdep.

Trac ticket: Core-64328.

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Props desrosj.

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[github-actions]

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