Fix kanban set previous neighbor

[paulmskim]

Description

This PR fixes setting previous neighbor for the kanban component.

Related Issue

Closes: #7104

Testing

Testing Requirements

• This needs to be tested in multiple browsers.
• This needs to be tested with both Gravity and Orbital themes.
• This needs to be tested with and without AJAX submission.
• This needs accessibility testing.
• This needs to be tested with different versions of PHP or database servers.
• This needs to be tested with different versions of WordPress.
• None of the above.

Testing Instructions

• Make movement changes to the kanban cards, ensure the retrieved updated kanban is correct based on the move
• Ensure the loaded kanban data on refresh is correct based on the previousNeighbor property

QA Tracking Spreadsheet

Screenshots

Checklist

• I've checked the error log to ensure my code doesn't throw any errors.
• I've added a changelog entry (if necessary) and updated the version number in 4 places, run npm install and committed package-lock.json (if necessary).
• I have added a "Documentation Needed" label to this PR (if necessary).

[speerface]

Tests are passing, and the switch to a deterministic result vs. the last-key fallback is a smart change, so I think this is good to merge. Let's just keep an eye on things to make sure something isn't borking elsewhere (but that should be pretty obvious in the UI). Thanks Paul!