fix(header): remove Shadow from Ionic Header (#30818)

Issue number: internal

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## What is the new behavior?
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this PR. -->
- The header should not have a shadow.
- Following the design experience, this shadow shouldn't exist. If
needed, the dev should use the divider prop

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->

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Co-authored-by: ionitron <hi@ionicframework.com>