fix: Use user_verification=discouraged for Reauth WebAuthn challenge.

[joelferrier]

When a fido2 compliant security key is used, and a PIN is configured, the user_verification=preferred value forces PIN authentication.

The Reauth API security key challenge is modeled on U2F which does not expose UV requirements. This change removes the PIN prompt requirement for Google Cloud customers with fido2 security keys and session length controls enabled which require security key reauth.

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Fixes #17975

[gemini-code-assist]

Code Review

This pull request updates the WebAuthn user verification parameter from "preferred" to "discouraged" in challenges.py and updates the corresponding test cases in test_challenges.py and test_webauthn_types.py to align with this change. There are no review comments, so I have no feedback to provide.