[webauthn] Make signature counters a MAY ? (#1590)

equalsJeffH has just created a new issue for https://github.com/w3c/webauthn:

== Make signature counters a MAY ? ==
Signature Counter section says:
> Authenticators SHOULD implement a signature counter feature.

However, we're not aware that anyone has ever gotten any utility from them.  (sez @agl)

in WebAuthn L3 we perhaps ought to update that section to downgrade it to a MAY and/or bluntly add the above statement.

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/1590 using your GitHub account


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