fix: correct button visited color

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR fixes a specificity change in #2109 -- normally the text colour of the button would be overwritten in the colours PHP, but with it moving to CSS variables, the :visited colour of the button block ends up always being white, often incorrectly.

See 1200550061930446-as-1206624531643238

How to test the changes in this Pull Request:

1. Add a set of linked button blocks to a post or page -- include:

• The default colours
• Custom colours using the palette (greys, or the primary, primary variation, or secondary variation colours)
• Fully custom colours using the colour pickers.

2. Navigate to Customizer > Colors, and change the Secondary colour to a light colour. This is used by the button block by default, and will change the button text to black.
3. Click on your links and come back to the page; note that the visited link of the default block's text turns white:

4. Apply the PR and run npm run build.
5. Confirm that the default block's text stays black.

6. Confirm the contrast on the other buttons still matches what you picked.
7. Try switching the Secondary colour back to a darker colour, and confirm that the visited button still looks good.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

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[chickenn00dle]

Looks good!

[matticbot]

🎉 This PR is included in version 1.85.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀