replace deprecated Text color in course settings
replace deprecated Text color in course settings

refs LS-3477
flag=none

test plan:
  - n/a

qa risk: low

Change-Id: Ia7a85e9a5fbaef68f23cb150d5fedca97903ab31
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/301429
Reviewed-by: Eric Saupe <e7287cf05ae62bc45889f17a9d34b8fae8d5afd1@instructure.com>
QA-Review: Eric Saupe <e7287cf05ae62bc45889f17a9d34b8fae8d5afd1@instructure.com>
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Product-Review: Davis Hyer <60fbffed049dcae9186e63e354cbc0b825bf5523@instructure.com>

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