Text Sets: make use of custom heading levels

[timarney]

Context

Updates cover text sets to ensure "primary" title gets assigned an H1 tag

Summary

Updates cover text sets

Relevant Technical Choices

To-do

User-facing changes

Testing Instructions

• This is a non-user-facing change and requires no QA

This PR can be tested by following these steps:

1). Create new story
2) Add each cover text set to a page
3) Ensure the primary title is showing as an H1 (Style -> Accessibility -> heading level)

Reviews

Does this PR have a security-related impact?

No

Does this PR change what data or activity we track or use?

No

Does this PR have a legal-related impact?

No

Checklist

• This PR addresses an existing issue and I have linked this PR to it in ZenHub
• I have tested this code to the best of my abilities
• I have verified accessibility to the best of my abilities (docs)
• I have verified i18n and l10n (translation, right-to-left layout) to the best of my abilities
• This code is covered by automated tests (unit, integration, and/or e2e) to verify it works as intended (docs)
• I have added documentation where necessary
• I have added a matching Type: XYZ label to the PR

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Fixes #10724

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