FED-3197 Fix mermaid diagram newlines in migration guide

[greglittlefield-wf]

Motivation

In the null safety migration guide prop requiredness section, what're supposed to be newlines in the mermaid diagram are rendering as \n.

Changes

Replace \n with <br> to fix newline rendering

Release Notes

Review

See CONTRIBUTING.md for more details on review types (+1 / QA +1 / +10) and code review process.

Please review:

QA Checklist

• [ ] Tests were updated and provide good coverage of the changeset and other affected code
• [ ] Manual testing was performed if needed
  • [ ] Steps from PR author:
    • Open docs in GitHub and verify the diagram's newlines render as intended: https://github.com/Workiva/over_react/blob/2a0ed809cdb99da18fa6d32d20697843c281855b/doc/null_safety/null_safe_migration.md#prop-requiredness-and-nullability
  • [ ] Anything falling under manual testing criteria outlined in CONTRIBUTING.md

Merge Checklist

While we perform many automated checks before auto-merging, some manual checks are needed:

• [ ] A Frontend Frameworks Design member has reviewed these changes
• [ ] There are no unaddressed comments - this check can be automated if reviewers use the "Request Changes" feature
• [ ] For release PRs - Version metadata in Rosie comment is correct

[aviary-wf]

Security Insights

No security relevant content was detected by automated scans.

Action Items

• Review PR for security impact; comment "security review required" if needed or unsure

• Verify aviary.yaml coverage of security relevant code

  Questions or Comments? Reach out on Slack: #support-infosec.

[greglittlefield-wf]

@Workiva/release-management-p