Site icon: Use the same text to describe the site icon requirments

[netweb]

When selecting an image to upload a site icon via the settings workflow your prompted with:

Suggested image dimensions: 512 × 512

If you upload a file less than those dimensions in my case I chose an existing 1200x300 image and I'm prompted with:

The selected image is smaller than 512px in height.

As such the string Suggested image dimensions: 512 × 512 is not entirely accurate as a requirement of at least 512 x512 is enforced, when using the customizer UI to add a site icon the following is displayed:

The Site Icon is used as a browser and app icon for your site. Icons must be square, and at least 512px wide and tall.

Suggesting we use the same text in both places.

[ocean90]

Removed in [33329].