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

.storybook/test-runner.ts

-import { TestRunnerConfig, waitForPageReady } from '@storybook/test-runner';
+import { getStoryContext, TestRunnerConfig, waitForPageReady } from '@storybook/test-runner';
+import { checkA11y, configureAxe, injectAxe } from 'axe-playwright';
 import { toMatchImageSnapshot } from 'jest-image-snapshot';
 import { getResetAnimationsCSS } from '../src/utils/test_utils';
 import { join, relative } from 'node:path';
@@ -27,6 +28,8 @@ const chartsFailureThreshold = 0.0018;
 
 const isChart = ({ title }) => title.startsWith('charts/');
 
+const isBaseComponent = ({ title }) => title.startsWith('base/');
+
 const getMatchOptions = (context) => {
   const failureThresholdType: Threshold = isChart(context)
     ? chartsFailureThresholdType
@@ -41,6 +44,29 @@ const getMatchOptions = (context) => {
 
 const fileExists = async (path) => !!(await stat(path).catch((e) => false));
 
+const runA11y = async (page, context) => {
+  const storyContext = await getStoryContext(page, context);
+
+  if (isBaseComponent(context) && !storyContext.parameters?.a11y?.disable) {
+    await configureAxe(page, {
+      rules: storyContext.parameters?.a11y?.config?.rules,
+    });
+
+    await checkA11y(
+      page,
+      '#storybook-root',
+      {
+        detailedReport: true,
+        detailedReportOptions: {
+          html: true,
+        },
+      },
+      false,
+      'v2'
+    );
+  }
+};
+
 // For now, we generate identifiers that match legacy storyshots-generated files so that Git
 // understands we are moving files, not creating new ones.
 const getSnapshotIdentified = (context) =>
@@ -57,6 +83,8 @@ const config: TestRunnerConfig = {
   async preVisit(page) {
     page.setViewportSize(DEFAULT_VIEWPORT_SIZE);
 
+    await injectAxe(page);
+
     // Wait until assets have finished loading. It is worth noting that we ran into several timing
     // issues while setting up Test Runner that were eventually addressed by the addition of an
     // arbitrary timeout below in the `postVisit` hook. That works around the fact that
@@ -111,6 +139,9 @@ const config: TestRunnerConfig = {
       storeReceivedOnFailure: true,
       ...getMatchOptions(context),
     });
+
+    // run accessibility tests
+    await runA11y(page, context);
   },
   tags: {
     skip: ['skip-visual-test'],