Fix: gift aid step should exclude long text heading in favor of modal header

[jonwaldstein]

Description

When migrating forms with gift aid, we need to modify the existing long text if it has the same heading as the modal header so there is not 2 instances of the same text.

Affects

The gift aid blocklongExplanation attribute

Visuals

n/a

Testing Instructions

• Migration a v2 form with with the "What is Gift Aid?" heading in the long text field
• in v3 form this should be removed from the long text block field.

Pre-review Checklist

• [ ] Acceptance criteria satisfied and marked in related issue
• [ ] Relevant @unreleased tags included in DocBlocks
• [ ] Includes unit tests
• [ ] Reviewed by the designer (if follows a design)
• [ ] Self Review of code and UX completed

[jonwaldstein]

@pauloiankoski this ready for review. I also thought about doing the reverse and removing the modal header - but I think we want that modal header by default. let me know what you think

[github-actions[bot]]

This PR is stale because it has been open 45 days with no activity. Stale PRs will NOT be automatically closed.

[jonwaldstein]

This might not be necessary, closing for now.