[webauthn] Fix the "aborted flag" reference (#1682)

nsatragno has just created a new issue for https://github.com/w3c/webauthn:

== Fix the "aborted flag" reference ==
https://github.com/whatwg/dom/pull/1027 removed the "abort flag" definition on the dom spec, which broke our spec (and others).

We should either have it reference the ["aborted" attribute](https://dom.spec.whatwg.org/#dom-abortsignal-aborted) or the ["aborted" concept](https://dom.spec.whatwg.org/#abortsignal-aborted), and update the language accordingly.

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/1682 using your GitHub account


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