fix: Update disabled normal button text color for a11y contrast

[ywyyu17]

Description

Issue

11b Button dropdown - Main action
Loading spinners in normal variant buttons have a color contrast ratio below 3:1.

Foreground color: #A9A9A9
Background color: #FFFFFF
Contrast ratio: 2.35:1

Fix

Update color to use grey500 instead of grey450

How has this been tested?

manual test
pending screenshot test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.63%. Comparing base (154aba5) to head (2b50171).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4861   +/-   ##
=======================================
  Coverage   97.63%   97.63%           
=======================================
  Files         957      957           
  Lines       31097    31097           
  Branches    11435    11435           
=======================================
  Hits        30361    30361           
  Misses        689      689           
  Partials       47       47           

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