fix(tab-button): use darker text to pass a11y (#27355)

Issue number: N/A

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- Changes the default Material Design tab color to
`$text-color-step-350` or `#a6a6a6`
- Changes the default iOS tab color to `$text-color-step-400` or
`#999999`
- Removes the axe skip in the e2e test

These were the minimum color changes needed to pass axe.

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Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Liam DeBeasi <liamdebeasi@users.noreply.github.com>

core/src/components/tab-button/test/a11y/tab-button.e2e.ts

@@ -7,8 +7,7 @@ configs().forEach(({ title, config }) => {
     test('should not have any axe violations', async ({ page }) => {
       await page.goto('/src/components/tab-button/test/a11y', config);
 
-      // TODO FW-3604
-      const results = await new AxeBuilder({ page }).disableRules('color-contrast').analyze();
+      const results = await new AxeBuilder({ page }).analyze();
       expect(results.violations).toEqual([]);
     });
   });