content: update outdated imagemin code samples #2523

What page(s) need to be updated?

• https://web.dev/serve-images-webp/
• https://web.dev/use-imagemin-to-compress-images/

Why is this update needed?

The code samples in these files has become outdated due breaking API changes in the latest version of the imagemin library. See the following related issues for additional information:

• Console shows "Images Optimized", but there are no images in output folder. imagemin/imagemin-webp#23
• Example in readme does not work with imagemin v7 imagemin/imagemin-mozjpeg#45