chore(breaking): update picker component heading level (#28725)

Issue number: N/A

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## What is the current behavior?
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The picker heading level is currently an `h2`, where the other component
headings are `h4`.

## What is the new behavior?
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this PR. -->

- Updates the heading level for Picker to `h4`. 

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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