Find a better name for "devices" in GetAvailablePublicKeyDevices

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From the documentation:
> The word "devices" is used broadly and can refer to individual authenticators (like a locked passkey provider or linked hybrid device), a group of authenticators on a transport (USB or hybrid QR code devices), or even an individual credential (in the case of credentials supplied by unlocked passkey providers).

We should choose a different word besides "devices" that isn't so confusing. "sources" probably works better. It was originally avoided because there is a term for _credential sources_ in the WebAuthn spec that means something else, but perhaps it wouldn't be confusing in this case.

I'm open to other alternatives too.

This would change the name of the `GetAvailablePublicKeyDevices()` method.

Start by locating the GetAvailablePublicKeyDevices() method in the specification and all references to it. Review the surrounding WebAuthn terminology, then settle on an unambiguous replacement name and update the method consistently; done means the API no longer uses the confusing “devices” term.

Written by the indexing model from the issue text.