Fix FullClusterRestartIT using lenient booleans with 6.0

This fixes a test failure where `FullClusterRestartIT` would mistakenly use
lenient booleans for a 6.0 version of ES, which were then rejected, causing test
failures.

See: https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+6.x+intake/572

qa/full-cluster-restart/src/test/java/org/elasticsearch/upgrades/FullClusterRestartIT.java

@@ -65,7 +65,7 @@
 public class FullClusterRestartIT extends ESRestTestCase {
     private final boolean runningAgainstOldCluster = Booleans.parseBoolean(System.getProperty("tests.is_old_cluster"));
     private final Version oldClusterVersion = Version.fromString(System.getProperty("tests.old_cluster_version"));
-    private final boolean supportsLenientBooleans = oldClusterVersion.onOrAfter(Version.V_6_0_0_alpha1);
+    private final boolean supportsLenientBooleans = oldClusterVersion.before(Version.V_6_0_0_alpha1);
     private static final Version VERSION_5_1_0_UNRELEASED = Version.fromString("5.1.0");
 
     private String index;