fix(#2632): Dialog Heading should use H2 rather than H3 (#2633)

* fix(#2632): Dialog Heading should use H2 rather than H3

Add a level prop to specify the Heading level for Dialog heading slot.

* fix(#2633): remove public API for dialog heading level

Not sure how much use a public API would get.

* fix(#2633): remove redundant Heading level declaration

Co-authored-by: Danni <[email protected]>

Author: Michael Jordan

packages/@react-spectrum/dialog/src/Dialog.tsx

@@ -63,7 +63,7 @@ function Dialog(props: SpectrumDialogProps, ref: DOMRef) {
   let slots = useMemo(() => ({
     hero: {UNSAFE_className: styles['spectrum-Dialog-hero']},
     header: {UNSAFE_className: styles['spectrum-Dialog-header']},
-    heading: {UNSAFE_className: classNames(styles, 'spectrum-Dialog-heading', {'spectrum-Dialog-heading--noHeader': !hasHeader}), ...titleProps},
+    heading: {UNSAFE_className: classNames(styles, 'spectrum-Dialog-heading', {'spectrum-Dialog-heading--noHeader': !hasHeader}), level: 2, ...titleProps},
     typeIcon: {UNSAFE_className: styles['spectrum-Dialog-typeIcon']},
     divider: {UNSAFE_className: styles['spectrum-Dialog-divider'], size: 'M'},
     content: {UNSAFE_className: styles['spectrum-Dialog-content']},