[upstream] [Table] CKEditor 5 editing table with rowspans removes header property

Problem/Motivation

When you edit a saved CKEditor 5 table with multiple column headers, and at least one of those columns contains a merged cell, then the right most header column loses the header property.

Steps to reproduce

1. Create a table using CKEditor 5 (see img 1)
2. Merge two of the cells in the first column
3. Highlight the first column and mark as a header
4. Highlight the second column and mark as a header
5. Save and view the page. See that the two header columns are properly marked up (see img 2)
6. Edit the page, notice that the right header column is no longer marked as a header (see img 3)

Proposed resolution

• https://github.com/ckeditor/ckeditor5/pull/17870

Remaining tasks

Test proposed resolution and if it helps then ensure it gets merged so that core can upgrade to it.

User interface changes

See issue summary