fix(VDivider): visiblity in container with dynamic height

[J-Sek]

Description

Assuming <v-divider vertical> makes sense only inside flexbox container, the height: auto allows align-self: stretch to take effect when container does not have explicit height.

Example in docs does not expose the issue, because d-flex gets additional explicit height, but in real life, we usually leave height to be dynamic and determined based on the children height.

After the change I did not notice any negative effects, but I may not know about use cases outside flexbox.

fixes #19827