v1.18: fix: send votes to the immediate next leader (backport of #2607)

[mergify]

Problem

Votes are always sent to the "forward leader" which is defined as the leader of the slot which is 2 slots beyond our latest poh clock slot. This means that a vote for the third slot of a given leader's slot span will always be sent to the following leader for inclusion in the first slot of that leader's slot span. This adds an unnecessary slot of latency to votes because the vote could potentially land in current leader's fourth slot.

Summary of Changes

• Send votes to the leaders of the next two slots to decrease vote latency

Fixes #

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This is an automatic backport of pull request #2607 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of f4e2fa9 has failed:

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Changes to be committed:
	modified:   core/src/voting_service.rs

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

[jstarry]

@AshwinSekar ran the data on testnet vote latencies before/after this change was merged to v2.0 and saw significant improvement in vote inclusion during the 4th slot of leader slot spans so I think we should backport this to v1.18 to improve vote latency on mainnet.

EDIT: link to testnet data: #1851 (comment)