Gov: prevent proposals moving from Expired to Queued due to Grace Period change

[eladmallel]

Based on an issue found by C4 as part of the DAO V3 audit: code-423n4/2023-07-nounsdao-findings#55.

In this part of the proposal state function:

} else if (block.timestamp >= proposal.eta + getProposalTimelock(ds, proposal).GRACE_PERIOD()) {
    return NounsDAOStorageV3.ProposalState.Expired;
} else {
    return NounsDAOStorageV3.ProposalState.Queued;
}

Notice that if GRACE_PERIOD changes (e.g. by upgrading the timelock), a proposal can move from the Expired state back to the Queued state. This might have the undesirable effect of a proposal the DAO thought would not be executed, being executed unexpectedly.

A straightforward solution would be to capture a proposal's expiration timestamp when it is queued, similar to how a proposal's ETA (when it can be executed) is captured.