Draft: feat(Text): Update default text token color for darkmode

What does this MR do?

Refines the text.color.default design token for darkmode to use color.neutral.100, subtly dimming it to improve the visual hierarchy and readability across the UI. This slight adjustment allows strong and heading tokens to stand out more effectively, enhancing their prominence and enabling clearer differentiation of key text. This improvement contributes to an overall better reading experience by providing a more balanced contrast.

Screenshots or screen recordings

Before	After
Markdown Test:
Visual Test:

See integration branch.

Test:

• http://localhost:9001/?path=/story/tokens-text--dark
• http://localhost:9001/?path=/story/base-markdown--documentation

Integration merge requests

• GitLab: Draft: GitLab UI integration test for text-toke... (gitlab!171801)
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

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Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.

Related to gitlab#474922 (closed), gitlab-org/foundations&6