"Iframe element has non-empty accessible name" [cae760]: use `tabindex`

[Jym77]

Replace "can be accessed by sequential focus navigation" with "non-negative tabindex". This align with the recent changes made in #1855 on the other rule with iframes and tabindexes.

Closes issue(s):

• closes #2015

Need for Call for Review: This will require a 1 week Call for Review (aligning phrasing without changing intend)

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[Jym77]

Call for review ends on February 16th.

[Jym77]

Call for review has ended. Merging.