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 <drobinson@livefyre.com>