Convert image diff background image to CSS

What does this MR do?

Replaces the trans_bg.gif image used for image diff background image with a CSS implementation of the same background image

Are there points in the code the reviewer needs to double check?

None

Why was this MR needed?

Uses CSS to replace the use of an image so that webpage rendering is slightly improved

What are the relevant issue numbers?

Closes #20500 (closed)

Screenshots (if relevant)

Before:

After:

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together