Release note: None

voters to non-voters

This commit updates `AdminRelocateRange` to use explicit atomic swaps of
voting replicas with non-voting replicas, that cockroachdb#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 cockroachdb#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

This commit removes the relocation logic used by the `mergeQueue` thus
far to align replica sets (added in cockroachdb#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 cockroachdb#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.

Release note: None