[EuiPanel] Change default to no shadow (best if eng-led; could be exceptions)

[JasonStoltz]

[mgadewoll]

ℹ️ @ryankeairns Some remaining open questions:

• EuiPanel
  • transparent panels can have border but with the new hasBorder default, any current use case would automatically have a border which is likely unwanted as they are mostly used as layout container
• EuiDraggable/EuiDroppable (usage only as out examples use panels)
  • specs expect that shadow and border are not combined
  • @ek-so mentioned that we should add shadow when in isDragging state which means that we should to remove hasBorder when isDragging
  • Is it expected to change border styles between states (e.g. non-dragging panel has a border, if it's dragging and should have shadow, the panel border should be removed)
• EuiPage / EuiPageTemplate
  • originally we have a shadow between sidebar and body - the changed panel default would result in the shadow being removed
    • @JoseLuisGJ mentioned that we likely want to preserve the shadow as seen in screenshots in https://github.com/elastic/platform-ux-team/issues/902