feat(Bootstrap): Remove color variable settings in _reboot.scss

What does this MR do?

In this MR I move the color settings on anchors from the Bootstrap _reboot file to typography and apply design token colors. Additionally I moved the styles for the text-decoration, to keep it all in one place.

I believe that the following implementation could work as well, is less code, and does not have to use :not():

// Setting colors on real anchors with an href,
// but not on placeholder links/named anchors (without href).
a:is([href]) {
  @apply gl-text-link gl-no-underline hover:gl-text-link hover:gl-underline;
}

I did not want to create bugs in consuming repositories and decided to stick to a clear refactoring in this MR.

To test this locally, I suggest to go to http://localhost:9001/?path=/docs/base-link--docs and remove the CSS Class gl-link from the HTML. Then, the typography.scss styles get activated. Alternatively, the components with the screenshot updates can be verified.

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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 #2821 (closed)