Vaadin-split-layout has 0 height inside vaadin-vertical-layout

[web-padawan]

Description

Despite the issue affects vaadin-split-layout, the actual problem is how we use CSS flexbox for both horizontal and vertical layout.

Because of that, we can't safely apply workarounds like flex: none (vaadin/platform#695):

• it would help for vertical layout (where we don't expect items to stretch vertically),
• but it will affect also horizontal layout (where we do expect items to stretch horizontally).

Marking this as requiring new major. Will consider creating an "epic" for layout revamp.

[web-padawan]

Note: added V24 label just so that we don't forget about it in the future. We aren't going to change layouts much in V24.