Make What's New feature image optional

What does this MR do and why?

Related to: gitlab-com/www-gitlab-com!103219 (merged)

This MR makes the image_url optional for "What's New" items.

Currently, the field is required and if there isn't a product image, we use the stage icon. This requirement looks worse in most cases than a content block that doesn't have an image at all.

Screenshots or screen recordings

Current:

Proposed:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.