Fix bad contrast on links in announcement bar for light mode

[HonkingGoose]

What browser are you using?

Firefox

Other browser name

No response

Describe the bug

The link text is unreadable in the "light mode"-variant of the announcement bar:

The links in the dark-mode variant of the announcement bar score AAA on the WCAG contrast checker in Firefox. So we only need to fix the light mode. 😄

Steps to reproduce

1. Go to our public site: https://docs.renovatebot.com/
2. Make sure you're viewing the light mode variant
3. Open your browser's inspector tools for contrast and hover over the link text in the announcement bar, see WCAG contrast fail.

Additional context

Official docs about customizing the colors:

• https://squidfunk.github.io/mkdocs-material/customization/
• https://squidfunk.github.io/mkdocs-material/setup/changing-the-colors/#customization

[HonkingGoose]

Maybe this PR has a clue on how we can fix our announcement bar?

• Set link color in dark mode to indigo by default squidfunk/mkdocs-material#5206