Use a single setting to enable/disable primary shard balance/rebalance

[bugmakerrrrrr]

Is your feature request related to a problem? Please describe

Today, we use settings cluster.routing.allocation.balance.prefer_primary and cluster.routing.allocation.rebalance.primary.enable to enable/disable primary shard balance and rebalance separately.

OpenSearch/server/src/main/java/org/opensearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java

Lines 320 to 330 in 9de21d1

	private void setPreferPrimaryShardBalance(boolean preferPrimaryShardBalance) {
	this.preferPrimaryShardBalance = preferPrimaryShardBalance;
	this.weightFunction.updateAllocationConstraint(INDEX_PRIMARY_SHARD_BALANCE_CONSTRAINT_ID, preferPrimaryShardBalance);
	this.weightFunction.updateAllocationConstraint(CLUSTER_PRIMARY_SHARD_BALANCE_CONSTRAINT_ID, preferPrimaryShardBalance);
	this.weightFunction.updateRebalanceConstraint(INDEX_PRIMARY_SHARD_BALANCE_CONSTRAINT_ID, preferPrimaryShardBalance);
	}
	private void setPreferPrimaryShardRebalance(boolean preferPrimaryShardRebalance) {
	this.preferPrimaryShardRebalance = preferPrimaryShardRebalance;
	this.weightFunction.updateRebalanceConstraint(CLUSTER_PRIMARY_SHARD_REBALANCE_CONSTRAINT_ID, preferPrimaryShardRebalance);
	}

But it is wired that we also enable the constraint INDEX_PRIMARY_SHARD_BALANCE_CONSTRAINT_ID in RebalanceConstraints when settings cluster.routing.allocation.balance.prefer_primary as true, by definition, it should be controlled by the rebalance configuration.

On the other hand, just turning on AllocationConstraints doesn't do much good, as it may still cause the primary shards to become unbalanced during rebalance.

Describe the solution you'd like

Use a single setting cluster.routing.allocation.balance.prefer_primary to control primary shard balance and rebalance, deprecate the setting cluster.routing.allocation.rebalance.primary.enable and remove it in OS 3.0.

Related component

ShardManagement:Placement

Describe alternatives you've considered

No response

Additional context

No response