Updates the gl-heading-* mixings to use the correct design token
for it's text color.

feat(Typography): Update gl-heading to use design token color

See merge request gitlab-org/gitlab-ui!4758



Merged-by: Scott de Jonge <sdejonge@gitlab.com>
Approved-by: Dan MH <dmizzi-harris@gitlab.com>
Approved-by: Scott de Jonge <sdejonge@gitlab.com>
Co-authored-by: Sascha Eggenberger <seggenberger@gitlab.com>

# [101.6.0](gitlab-org/gitlab-ui@v101.5.0...v101.6.0) (2024-10-31)

### Features

* **Typography:** Update gl-heading to use design token color ([539409e3](gitlab-org/gitlab-ui@539409e3))