[BUG] Only show button drop shadow in att. lvl 3 on light background

[RasmusKjeldgaard]

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Describe the bug

When introducing the new att. lvl. 3 button design, the shadow was implemented on all button attention levels on light background, because of a misunderstanding of the design spec:

z2 elevation is only meant for att. lvl. 3.

Which Kirby version was used?

Unreleased as of writing this, introduced here: #2735

What was the expected behavior?

Only att. lvl. 3 on light background has shadow, everything else is 'flat' design.

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