Backport periodic flush count to v6.3.0

Relates #29360

Author: dnhatn

rest-api-spec/src/main/resources/rest-api-spec/test/indices.flush/10_basic.yml

@@ -25,8 +25,8 @@
 ---
 "Flush stats":
   - skip:
-      version: " - 6.99.99"
-      reason: periodic flush stats is introduced in 7.0
+      version: " - 6.2.99"
+      reason: periodic flush stats is introduced in 6.3.0
   - do:
       indices.create:
         index: test

server/src/main/java/org/elasticsearch/index/flush/FlushStats.java

@@ -114,7 +114,7 @@ static final class Fields {
     public void readFrom(StreamInput in) throws IOException {
         total = in.readVLong();
         totalTimeInMillis = in.readVLong();
-        if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (in.getVersion().onOrAfter(Version.V_6_3_0)) {
             periodic = in.readVLong();
         }
     }
@@ -123,7 +123,7 @@ public void readFrom(StreamInput in) throws IOException {
     public void writeTo(StreamOutput out) throws IOException {
         out.writeVLong(total);
         out.writeVLong(totalTimeInMillis);
-        if (out.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {
+        if (out.getVersion().onOrAfter(Version.V_6_3_0)) {
             out.writeVLong(periodic);
         }
     }