Fire the change event consistently

[dgrammatiko]

Pull Request for Issue #37640 .

Summary of Changes

• Call the Custom Element function setValue instead of manually updating the value

Testing Instructions

• Edit an article and go to the images tab
• Open the inspector and inspect the intro image input (the one left to the select button bellow the image)
• type in the browser's console $0.addEventListener('change', (e) => console.log(e))
• Select an image, check that a change event was recorded in the console
• Clear the field, check the recorded event
• Select another image and check that the event was recorded again

Actual result BEFORE applying this Pull Request

No event on image selection

Expected result AFTER applying this Pull Request

Documentation Changes Required

No this is a bug

BTW this is already fixed in 4.2 with #37654 and merging this will not end up in conflicts when syncing the 2 repos.