Bump gulp-imagemin from 5.0.3 to 9.1.0

Bumps [gulp-imagemin](https://github.com/sindresorhus/gulp-imagemin) from 5.0.3 to 9.1.0.
- [Release notes](https://github.com/sindresorhus/gulp-imagemin/releases)
- [Commits](sindresorhus/gulp-imagemin@v5.0.3...v9.1.0)

---
updated-dependencies:
- dependency-name: gulp-imagemin
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]