Rename Feature Policy directive for WebAuthn

[bershanskiy]

Feature Policy directive controlling WebAuth was renamed from
publickey-credentials to publickey-credentials-get.
Sources:
Feature Policy change:
w3c/webappsec-permissions-policy#370
WebAuthn specification change:
w3c/webauthn#1394
Chrome already updated the name:
https://crbug.com/993007#c9

Fortunately, this directive wasn't shipped anywhere yet, so there is no actual compat data to be updated. Even Chromium bug number is still relevant.

Relevant sprint issue: mdn/sprints#3214

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[Elchi3]

Thanks, I've moved the article as well. 👍

[bershanskiy]

Thanks, I closed mdn/sprints#3214 as well.