v2.0: gossip: ignore retransmitter signatures when comparing duplicate shreds (backport of #2673)

[mergify]

Problem

#1271 ignores the retransmitter signature when comparing shreds in order to generate a duplicate proof.

We need to do the same when verifying a duplicate proof received from gossip.

Summary of Changes

Ignore retransmitter signature when verifying a duplicate proof received through gossip

---

This is an automatic backport of pull request #2673 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	modified:   gossip/src/duplicate_shred.rs
	modified:   ledger/src/shred/merkle.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   ledger/src/shred.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[AshwinSekar]

This change fixes the duplicate blocks observed on testnet.

When checking if 2 shreds constitute a duplicate, we compare the payloads for a mismatch.
In the new shred format, a signature from the retransmitter is attached to the payload. These bits should not be a part of the comparison, as it is valid to receive the same shred from multiple sources.

Currently on testnet if we receive a duplicate proof for this situation via gossip, we erroneously mark the block as duplicate.