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.

Screenshots or screen recordings

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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

src/vendor/bootstrap/scss/_reboot.scss

@@ -39,8 +39,7 @@ article, aside, figcaption, figure, footer, header, hgroup, main, nav, section {
 // Body
 //
 // 1. Remove the margin in all browsers.
-// 2. As a best practice, apply a default `background-color`.
-// 3. Set an explicit initial text-align value so that we can later use
+// 1. Set an explicit initial text-align value so that we can later use
 //    the `inherit` value on things like `<th>` elements.
 
 body {
@@ -49,9 +48,7 @@ body {
   @include font-size($font-size-base);
   font-weight: $font-weight-base;
   line-height: $line-height-base;
-  color: $body-color;
   text-align: left; // 3
-  background-color: $body-bg; // 2
 }
 
 // Future-proof rule: in browsers that support :focus-visible, suppress the focus outline