After #7592, @cmwaters noticed that the logic for re-using old timestamps for proposals may not work with proposer-based timestamps. This change removes the logic to re-use old proposal timestamps since it is no longer correct. Two proposals with different timestamps can no longer be treated as equivalent. Signing a proposal that only differs by timestamp in the new algorithm can be thought of as roughly equivalent to signing a proposal that only differs by `BlockID` in the old scheme.
I also investigated the codebase and checked for any place we updated a timestamp using the pattern `(Timestamp = |Timestamp: )` and saw no additional places where we are updating the timestamp of a proposal message.
Here is the output of that search:
```
privval/file.go:372: vote.Timestamp = timestamp
privval/file.go:453: lastVote.Timestamp = now
privval/file.go:454: newVote.Timestamp = now
internal/test/factory/commit.go:25: Timestamp: now,
internal/test/factory/vote.go:34: Timestamp: time,
internal/consensus/state.go:2261: Timestamp: cs.voteTime(),
internal/consensus/state.go:2286: vote.Timestamp = v.Timestamp
light/detector.go:414: ev.Timestamp = common.Time
light/detector.go:418: ev.Timestamp = trusted.Time
types/block.go:616: Timestamp: ts,
types/block.go:725: Timestamp: cs.Timestamp,
types/block.go:736: cs.Timestamp = csp.Timestamp
types/block.go:800: Timestamp: commitSig.Timestamp,
types/evidence.go:84: Timestamp: blockTime,
types/evidence.go:190: dve.Timestamp = evidenceTime
types/evidence.go:202: Timestamp: dve.Timestamp,
types/evidence.go:228: Timestamp: pb.Timestamp,
types/evidence.go:382: Timestamp: %v}#%X`,
types/evidence.go:491: l.Timestamp = evidenceTime
types/evidence.go:517: Timestamp: l.Timestamp,
types/evidence.go:546: Timestamp: lpb.Timestamp,
types/evidence.go:722: Timestamp: time,
types/vote.go:80: Timestamp: vote.Timestamp,
types/vote.go:216: Timestamp: vote.Timestamp,
types/vote.go:240: vote.Timestamp = pv.Timestamp
types/test_util.go:27: Timestamp: now,
types/proposal.go:44: Timestamp: tmtime.Now(),
types/proposal.go:132: pb.Timestamp = p.Timestamp
types/proposal.go:157: p.Timestamp = pp.Timestamp
types/canonical.go:49: Timestamp: proposal.Timestamp,
types/canonical.go:62: Timestamp: vote.Timestamp,
test/e2e/runner/evidence.go:186: Timestamp: evTime,
```
William Banfield
7bc3a7274a