Temporarily set BWC version to 6.0.0 for seq. no

There is not yet a BWC layer in sequence numbers. This commit sets the
BWC version to 6.0.0 for the BWC and rolling upgrade tests until this
BWC layer is built.

qa/backwards-5.0/build.gradle

@@ -18,7 +18,7 @@ integTest {
   cluster {
     numNodes = 2
     numBwcNodes = 1
-    bwcVersion = "5.1.0-SNAPSHOT"
+    bwcVersion = "6.0.0-alpha1-SNAPSHOT"
     setting 'logger.org.elasticsearch', 'DEBUG'
   }
 }

qa/rolling-upgrade/build.gradle

@@ -25,7 +25,7 @@ task oldClusterTest(type: RestIntegTestTask) {
   mustRunAfter(precommit)
   cluster {
     distribution = 'zip'
-    bwcVersion = '5.1.0-SNAPSHOT' // TODO: either randomize, or make this settable with sysprop
+    bwcVersion = '6.0.0-alpha1-SNAPSHOT' // TODO: either randomize, or make this settable with sysprop
     numBwcNodes = 1
     numNodes = 2
     clusterName = 'rolling-upgrade'