Bump gulp-imagemin from 6.2.0 to 7.0.0 in /arctic/static/arctic

Bumps [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin) from 6.2.0 to 7.0.0.
- [Release notes](https://github.com/sindresorhus/gulp-imagemin/releases)
- [Commits](sindresorhus/gulp-imagemin@v6.2.0...v7.0.0)

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