Update version in serialization of t-test agg usage stats (#54746)

Update version in the t-test agg usage stats serialization
after backport to 7.8.0

Relates to #53692

Author: imotov

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/analytics/action/AnalyticsStatsAction.java

@@ -147,7 +147,7 @@ public NodeResponse(StreamInput in) throws IOException {
                 stringStatsUsage = 0;
                 topMetricsUsage = 0;
             }
-            if (in.getVersion().onOrAfter(Version.V_8_0_0)) { // Will drop to 7.8.0 after backport
+            if (in.getVersion().onOrAfter(Version.V_7_8_0)) {
                 ttestUsage = in.readVLong();
             } else {
                 ttestUsage = 0;
@@ -165,7 +165,7 @@ public void writeTo(StreamOutput out) throws IOException {
                 out.writeVLong(stringStatsUsage);
                 out.writeVLong(topMetricsUsage);
             }
-            if (out.getVersion().onOrAfter(Version.V_8_0_0)) { // Will drop to 7.8.0 after backport
+            if (out.getVersion().onOrAfter(Version.V_7_8_0)) {
                 out.writeVLong(ttestUsage);
             }
         }