aria heading levels - must set

[cliffbert]

Per discussion w/ baseline group: https://github.com/atbcb/ICTTestingBaseline/issues/406

Prior to WAI-ARIA 1.2 the heading role had an implicit default aria-level value of 2. As of WAI-ARIA 1.2 this property must be explicitly set.

https://www.w3.org/TR/wai-aria-1.2/#heading