Styles: align elevation styles for code and design

Which issue does this PR close?

This PR closes #2960

Re-introduced elevation z8 with the current value of z4 and updated the z4 value. This also fixes the missing shadow on Menu (z4).

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[x] Make sure you have implemented tests following the guidelines in: "The good: Test".
[ ] Make sure you have updated the cookbook with examples and showcases (for bug fixes, enhancements & new components).

[ ] Determine if your changes are a fix, feature or breaking-change, and add the matching label to your PR. If it is tooling, dependency updates or similar, add ignore-for-release.
[x] Do a self-review.
[x] Request that the changes are code-reviewed
[ ] Request that the changes are UX reviewed (only necessary if your PR introduces visual changes)

When the pull request has been approved it will be merged to develop by Team Kirby.