Use CSS grid for table insertion drop-down

[oleq]

📝 Provide a description of the improvement

The feature uses flex instead. This is probably due to the fact that when first implemented there was no browser-wide support for display: grid or it was buggy. Let's clean it up.

A follow-up of https://github.com/ckeditor/ckeditor5/pull/12439#pullrequestreview-1102150332.

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