Update gulp-imagemin requirement from 4.1.0 to 5.0.3
Update gulp-imagemin requirement from 4.1.0 to 5.0.3

Updates the requirements on [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin) to permit the latest version.
- [Release notes](https://github.com/sindresorhus/gulp-imagemin/releases)
- [Commits](https://github.com/sindresorhus/gulp-imagemin/commits/v5.0.3)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
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