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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BREAKING.md

@@ -66,7 +66,7 @@ This section details the desktop browser, JavaScript framework, and mobile platf
       +  background: red;
       }
       ```
-<h2 id="version-8x-picker">Picker</h2>
+<h4 id="version-8x-picker">Picker</h4>
 
 - `ion-picker` and `ion-picker-column` have been renamed to `ion-picker-legacy` and `ion-picker-legacy-column`, respectively. This change was made to accommodate the new inline picker component while allowing developers to continue to use the legacy picker during this migration period.
   - Only the component names have been changed. Usages such as `ion-picker` or `IonPicker` should be changed to `ion-picker-legacy` and `IonPickerLegacy`, respectively.