fix(notification): background-warning-token to use yellow10

[ariellalgilmore]

Closes #17586

update notification-background-warning-token to use yellow10

Changelog

Changed

• using yellow10 now instead of mixing colors

Testing / Reviewing

• go to the playground of notification stories
• turn on low-contrast
• set kind to warning
• confirm correct color in the background

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.03%. Comparing base (6bd017d) to head (ddf5ccd).
Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17621   +/-   ##
=======================================
  Coverage   77.03%   77.03%           
=======================================
  Files         408      408           
  Lines       13993    13993           
  Branches     4349     4348    -1     
=======================================
  Hits        10779    10779           
- Misses       3041     3042    +1     
+ Partials      173      172    -1     

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