Flag aria-level values > 6

[straker]

After doing some testing, it seems aria-level values > 6 are handled inconsistently across screen readers. For VO and NVDA, any positive value is allowed and applied to the heading. But in JAWS and Talkback they are not allowed and the aria-level will be announced as the default value (2 for role=heading elements, the heading level for H elements).

We decided we should create a new check for aria-valid-attr-value that looks specifically at the aria-level and if it's greater than 6 mark the element as Needs Review with a message stating lack of browser support.

See https://codepen.io/straker/pen/jOBjNNe

[padmavemulapati]

Validated with the latest axe-core develop branch code (dated 15th July, 2021)
When aria-level value passed as greater than 6, either in div or in heading (does not matter where we are passing),
its showing message as
aria-level values greater than 6 are not supported in all screenreader and browser combinations
also for this , aria-valid-attr-value is appearing in needs review
test script:

<div role="heading" aria-level="12" id="target">Twelve</div>
    <h1 aria-level="10" id="target">Ten</h1>

[straker]

No docs