rename zones to serverSets to avoid terminology conflict (#10679)

we are bringing in availability zones, we should avoid
zones as per server expansion concept.

docs/zh_CN/distributed/DESIGN.md

@@ -113,19 +113,19 @@ minio server http://host{1...32}/export{1...32} http://host{5...6}/export{1...8}
 MinIO根据每个区域的可用空间比例将新对象放置在区域中。以下伪代码演示了此行为。
 ```go
 func getAvailableZoneIdx(ctx context.Context) int {
-        zones := z.getZonesAvailableSpace(ctx)
-        total := zones.TotalAvailable()
+        serverSets := z.getServerSetsAvailableSpace(ctx)
+        total := serverSets.TotalAvailable()
         // choose when we reach this many
         choose := rand.Uint64() % total
         atTotal := uint64(0)
-        for _, zone := range zones {
+        for _, zone := range serverSets {
                 atTotal += zone.Available
                 if atTotal > choose && zone.Available > 0 {
                         return zone.Index
                 }
         }
         // Should not happen, but print values just in case.
-        panic(fmt.Errorf("reached end of zones (total: %v, atTotal: %v, choose: %v)", total, atTotal, choose))
+        panic(fmt.Errorf("reached end of serverSets (total: %v, atTotal: %v, choose: %v)", total, atTotal, choose))
 }
 ```