Rename `T::MaxSilenceTime` to reflect actual usage.

[clangenb]

It is only checked to see if the remote attestation timestamp is too old, hence we should rename it accortingly.

Maybe:

• AttestationTimeout
• AttestationValidity
• MaxAttestationRenewalPeriod
• AttestationRenewalPeriod ?

[clangenb]

Closed by #198