Adjust BWC version on settings upgrade test (elastic#33650)

The skip_unavailable setting did not exist until 6.1.0. This means that
we need to skip this test on versions prior to 6.1.0. We need to use
this setting because otherwise we will fail startup without it (since we
are not setting up a real remote cluster connection). This commit adds a
skip for all versions prior to 6.1.0.

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

@@ -42,6 +42,7 @@
 public class FullClusterRestartSettingsUpgradeIT extends AbstractFullClusterRestartTestCase {
 
     public void testRemoteClusterSettingsUpgraded() throws IOException {
+        assumeTrue("skip_unavailable did not exist until 6.1.0", getOldClusterVersion().onOrAfter(Version.V_6_1_0));
         assumeTrue("settings automatically upgraded since 6.5.0", getOldClusterVersion().before(Version.V_6_5_0));
         if (isRunningAgainstOldCluster()) {
             final Request putSettingsRequest = new Request("PUT", "/_cluster/settings");