release-21.1: kvserver: get rid of bespoke relocation logic used by the mergeQueue

[aayushshah15]

Backport 3/3 commits from #62631.

/cc @cockroachdb/release

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This PR contains two main commits:

kvserver: update AdminRelocateRange to leverage explicit swaps of
voters to non-voters

This commit updates AdminRelocateRange to use explicit atomic swaps of
voting replicas with non-voting replicas, that #58627 initially added
support for. The patch does so by generalizing behavior that's already
exercised by the replicateQueue when it decides to rebalance replicas.
See #61239.

This allows us, in the next commit, to remove bespoke relocation logic
that's used by the mergeQueue to align replica sets for the sake of a
range merge.

Release note: None

kvserver: get rid of bespoke relocation logic used by the mergeQueue

This commit removes the relocation logic used by the mergeQueue thus
far to align replica sets (added in #56197). This logic previously
existed in order to allow us to align the replica sets of a pair of
ranges (which is required for the range merge to proceed), while
avoiding redundant data movement.

Before #58627 and the previous commit in this PR, AdminRelocateRange
couldn't be directly used by the mergeQueue under various configurations
of the LHS and RHS ranges. Furthermore, even when it could be used, it
would involve redundant data movement. This all required us to compute
relocation targets for the RHS of a merge separately, above the call to
AdminRelocateRange, for the range merge to proceed.

All these limitations have been resolved by the previous commit which
teaches AdminRelocateRange to promote non-voters and demote voters
when needed, and the aforementioned bespoke relocation logic is no
longer needed.

Resolves #62370

Release note: None

[cockroach-teamcity]

This change is