Fix type of AuthenticatorAttestationResponseJSON.publicKeyAlgorithm

Fixes #2065.

`AuthenticatorAttestationResponseJSON` was added in the L3 drafts, so
we can easily change
`AuthenticatorAttestationResponseJSON.publicKeyAlgorithm` to type
`long` (or `COSEAlgorithmIdentifier`) since L3 isn't formally released
yet. `AuthenticatorAttestationResponseJSON.publicKeyAlgorithm` is also
in output (covariant) position, so changing its type to be more
restrictive is even backwards compatible.

index.bs

@@ -1644,7 +1644,7 @@ that are returned to the caller when a new credential is created, or a new asser
         // algorithm then the public key must be parsed directly from
         // attestationObject or authenticatorData.
         Base64URLString publicKey;
-        required long long publicKeyAlgorithm;
+        required COSEAlgorithmIdentifier publicKeyAlgorithm;
         // This value contains copies of some of the fields above. See
         // section “Easily accessing credential data”.
         required Base64URLString attestationObject;