Allocate searchable snapshots with the balancer (#54889)

Today the shards of searchable snapshots are allocated with a naive
`ExistingShardsAllocator` which selects the first valid node for each shard.
Thanks to #54729 we can now allow these shards to fall through to the balanced
shards allocator so that they are allocated in a more balanced fashion.

Relates #50999

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/SearchableSnapshotAllocator.java

@@ -42,14 +42,14 @@ public void allocateUnassigned(
         if (allocateUnassignedDecision.getAllocationDecision() == AllocationDecision.YES) {
             if (shardRouting.primary() && shardRouting.recoverySource().getType() == RecoverySource.Type.EXISTING_STORE) {
                 // we don't care what the allocation ID is since we know that these shards cannot really be stale, so we can
-                // safely ignore the allocation ID with a forced-stale allocation
+                // safely ignore the allocation ID with a forced-stale allocation and allow this shard to fall through to the balanced
+                // shards allocator
                 unassignedAllocationHandler.updateUnassigned(
                     shardRouting.unassignedInfo(),
                     RecoverySource.ExistingStoreRecoverySource.FORCE_STALE_PRIMARY_INSTANCE,
                     allocation.changes()
                 );
             }
-            unassignedAllocationHandler.initialize(allocateUnassignedDecision.getTargetNode().getId(), null, 0L, allocation.changes());
         } else {
             unassignedAllocationHandler.removeAndIgnore(allocateUnassignedDecision.getAllocationStatus(), allocation.changes());
         }