#7466848 Marketplace: Update alt & display name on submission success page

Proposed Changes

According to comments here, we should change these:

• Show the display_name instead of username in the Hello, message.
• Remove the alt on the submissionSuccessImage image.

Testing Instructions

1. Go to /marketplace/submission-success?returning=true
2. Check showed name.
3. Open the developers console and check for the alt on the image (should be empty)

Screenshot

Before	After

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?

Fixes #66275
Fixes #66273