Fix BWC issue for PreSyncedFlushResponse

I misunderstood how the bwc versions works. If we backport to 5.x, we
need to backport to all supported 6.*.  This commit corrects the BWC
versions for PreSyncedFlushResponse.

Relates #29103

Author: dnhatn

server/src/main/java/org/elasticsearch/indices/flush/SyncedFlushService.java

@@ -561,19 +561,11 @@ static final class PreSyncedFlushResponse extends TransportResponse {
         }
 
         boolean includeNumDocs(Version version) {
-            if (version.major == Version.V_5_6_8.major) {
-                return version.onOrAfter(Version.V_5_6_8);
-            } else {
-                return version.onOrAfter(Version.V_6_2_2);
-            }
+            return version.onOrAfter(Version.V_5_6_8);
         }
 
         boolean includeExistingSyncId(Version version) {
-            if (version.major == Version.V_5_6_9.major) {
-                return version.onOrAfter(Version.V_5_6_9);
-            } else {
-                return version.onOrAfter(Version.V_6_3_0);
-            }
+            return version.onOrAfter(Version.V_5_6_9);
         }
 
         @Override