reference CTAP2.1 PS spec and fix broken link

[equalsJeffH]

webauthn L2 Rec references the old CTAP v2.0-ps-20190130 spec, and thus the link to "large, per-credential blobs" does not work (the latter is a webauthn L2 Rec errata item (I dunno if it is worth fixing)).

we need to update the webauthn L3 working draft should link to https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-20210615.html

the link to [=§6.2. Responses=] in the CTAP2.1 spec no longer works because we removed that entire section from the CTAP spec (doh). the two other links to the CTAP spec continue to work.