[Drag & Drop blocks] Vertically align the dropping insertion point when is over the dragged block

## Description

Following https://github.com/WordPress/gutenberg/pull/39891#pullrequestreview-927431464, it would be great if the dropping insertion point is centered when is displayed in the position of the block being dragged.

<img src=https://user-images.githubusercontent.com/14905380/161055754-a23148e9-0cef-4dd2-b8e3-694ab70230a0.jpeg width=350>