We rewrote the load-based rebalancing scheme in #65379 and #72296. We'll need to version gate this implementation of the StoreRebalancer for mixed version settings and probably have mixed version clusters behave identically to 21.2 clusters in terms of their load-based rebalancing behavior.

To this end, we should introduce a DeprecatedStoreRebalancer that is the 21.2 implementation and only switch to the new implementation after the version upgrade is finalized.

Jira issue: CRDB-13240