Build Tools: Update grunt-contrib-imagemin to v1.0.0

This updates `imagemin` to 4.0.0, which fixes an error when running `grunt imagemin:core`.

Props netweb.

Fixes #35003.


Built from https://develop.svn.wordpress.org/trunk@35859


git-svn-id: https://core.svn.wordpress.org/trunk@35823 1a063a9b-81f0-0310-95a4-ce76da25c4cd