fix: make captions styles more specific

[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 small PR fixes some consistency issues between the <figcaption> tag and the .wp-caption-text class for the purposes of consistency in older migrated content.

WordPress is inserting this CSS, which is making these two selectors behave differently in the theme:

:root :where(.wp-block-image figcaption) {
  color: #555;
  font-size: 13px;
  text-align: center;
}

This PR overrides that so they display the same.

How to test the changes in this Pull Request:

1. Apply this PR.
2. Add an image block with a caption, and below that, add a paragraph block with the CSS class .wp-caption-text.
3. Compare them on the front-end and in the editor; confirm they use the same colour and same font size.

4. Test a couple different themes to make sure the styles are consistent (eg. Katharine should have a little accent; Sacha should be bold, etc):

6. Try switching the heading font under Customizer > Typography and confirm that both caption-types pick it up.
   

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?

[leogermani]

The paragraph still looks a bit bigger on my end

[laurelfulford]

Thanks @leogermani! 🙌

[github-actions]

Hey @laurelfulford, good job getting this PR merged! 🎉

Now, the needs-changelog label has been added to it.

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Thank you! ❤️

[matticbot]

🎉 This PR is included in version 2.4.1-alpha.1 🎉

The release is available on GitHub release

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[matticbot]

🎉 This PR is included in version 2.4.2 🎉

The release is available on GitHub release

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