text: Title: Update notification email header image
  Body: ### Issue Summary
  
  When receiving an email notification from Wagtail, the image header appears to use the old logo and green/teal background.
  
  <img width="619" alt="Screenshot 2024-09-27 at 11 49 07 AM" src="https://github.com/user-attachments/assets/e907f1cb-e43d-401d-b7fe-6591fd6cd51d">
  
  ### Steps to Reproduce
  
  1. On a Wagtail site, submit a page for moderation in a workflow.
  2. A moderator should receive a notification email from Wagtail
  3. If applicable, click 'load images' in your email client.
  
  -   I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes
  
  ### Technical details
  
  -   Python version: 3.12.4
  -   Django version: 5.0.9
  -   Wagtail version: 6.1.3
  -   Browser version: N/A
  
  ### Working on this
  
  Relevant image file is located at: https://github.com/wagtail/wagtail/blob/main/wagtail/admin/static_src/wagtailadmin/images/email-header.jpg
  
  I'd expect an image with the new Wagtail logo (see https://github.com/wagtail/wagtail/pull/11756) and a dark purple background, like Wagtail uses these days.
  
  The base notifications template: https://github.com/wagtail/wagtail/blob/abcb2da37259c16d1efd16d7233794f00bb29680/wagtail/admin/templates/wagtailadmin/notifications/base.html