Bump imagemin from 8.0.1 to 9.0.0 (fixes #238)

Bumps [imagemin](https://github.com/imagemin/imagemin) from 8.0.1 to 9.0.0.
- [Release notes](https://github.com/imagemin/imagemin/releases)
- [Commits](imagemin/imagemin@v8.0.1...v9.0.0)

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

Change-Id: Ic6028fd09395c958268ca61c167d96d7d3a1df9d
Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package.json

@@ -19,7 +19,7 @@
     "grunt-sass": "^4.0.0",
     "grunt-shell": "^4.0.0",
     "grunt-terser": "^2.0.0",
-    "imagemin": "^8.0.1",
+    "imagemin": "^9.0.0",
     "optipng-bin": "^9.0.0",
     "sass": "^1.44.0",
     "vinyl-fs": "^4.0.0"