Remove declaration_text from forms (part 1/2)

[lfdebrux]

What problem does this pull request solve?

The declaration_text attribute on the forms model has been replaced with the declaration_markdown attribute and deprecated. It is no longer read in forms-runner or read or updated in forms-admin.

Removing the deprecated attribute completely will simplify the form model and reduce the possibility of confusion.

Removing the attribute from the forms model will not remove it from the form document model or change existing form document contents, but will stop if from being present in new form documents.

This change will prevent declaration_text being copied from a form document to a form model when copying a form form or reverting a draft form, but there are no form documents in the production database which have declaration_text but not declaration_markdown that could cause issues ¹.

This PR tells the form model to ignore the declaration_text column, so that it can be safely removed in PR #3108.

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• Ensure that you consider the wider context.
• Does it work when run on your machine?
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• Are there all the unit tests needed?
• Do the end to end tests need updating before these changes will pass?
• Has all relevant documentation been updated?

Footnotes

1. This was verified with the following SQL query:

    gds aws forms-prod-support -- forms data_api -d forms-admin -s $'SELECT form_id, tag, version, language, content->\'declaration_text\' AS declaration_text, content->\'declaration_markdown\' AS declaration_markdown FROM form_documents WHERE content @@ \'($.declaration_text != null && $.declaration_text != "") && ($.declaration_markdown == null || $.declaration_markdown == "")\';'
   

   Prior to https://github.com/govuk-forms/forms-runner/pull/2334, the only situation when the declaration_text attribute was read was when declaration_markdown was blank.

   There are two forms in the production database where declaration_markdown is blank and declaration_text is present, however, these were reported as bugs by a form creator, as they had wanted to remove the form declaration. ↩

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