Hotfix: Propagate data changes for Block Grid Area Types

[nielslyngsoe]

Making the Block Grid Areas Property Editor UI propagate changes to the property value, so it can be persisted.

Fixes umbraco/Umbraco-CMS#16640

Description

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Motivation and context

How to test?

Screenshots (if appropriate)

Checklist

• If my change requires a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[iOvergaard]

I have tried creating a layout with 4/8 areas and saved that, and that was persisted. Then tried adding blocks to those areas in a document, and those got persisted as well. LGTM.