Update semantics header and headingLevel behavior on iOS and Android

The header semantics property now behaves as a no-op on iOS and Android. To declare a section heading for accessibility purposes on these platforms, use the headingLevel property with a value greater than 0.

Historically, Flutter used the boolean header semantics property to denote headings on platform implementations:

• On Android, a header value of true mapped to View.setHeading(true).
• On iOS, a header value of true mapped to the UIAccessibilityTraitHeader accessibility trait.

However, setHeading and UIAccessibilityTraitHeader represent headings (equivalent to section headers/headings), which are best represented by a heading level. In contrast, "header" on these platforms often represents a banner or an app bar (such as SliverAppBar or AppBar), creating confusion and mismatch between platform APIs and Flutter properties.

With this change:

• The header semantics property behaves as a no-op on iOS and Android. It remains available in the API and can still be used in the future if similar APIs are provided.
• The headingLevel property is updated so that setting it to a value greater than 0 maps directly to View.setHeading(true) on Android and UIAccessibilityTraitHeader on iOS. On iOS 13+, it also maps to accessibilityHeadingLevel.

If your code previously used Semantics(header: true, ...) or SemanticsProperties(header: true, ...) to declare headings, migrate your code to use headingLevel: 1 (or another integer greater than 0).

Note that while setting headingLevel to any value greater than 0 declares a heading on Android and iOS, other platforms (such as the web) treat the specific heading level number differently. For example, on the web, values 1 through 6 map to the corresponding <h1> through <h6> HTML elements.

Code before migration:

Semantics(
  header: true,
  child: Text('Section Title'),
)

Code after migration:

Semantics(
  headingLevel: 1,
  child: Text('Section Title'),
)

Similarly, if you used SemanticsProperties:

Code before migration:

SemanticsProperties(
  header: true,
)

Code after migration:

SemanticsProperties(
  headingLevel: 1,
)

Landed in version: 3.45.0-0.1.pre
In stable release: TBD

API documentation:

• Semantics
• SemanticsProperties
• SemanticsConfiguration

Relevant issues:

• Issue 175416

Relevant PRs:

• PR 186916
• PR 175416