Set BWC builds for 6.x to use JDK 11

The BWC builds for the 6.x branch should be using JDK 11. This commit
fixes the BWC builds to specify that they use JDK 11 instead of JDK 10
which is now incompatible with the 6.x build.

distribution/bwc/build.gradle

@@ -157,8 +157,10 @@ subprojects {
       environment('JAVA_HOME', getJavaHome(it, 8))
     } else if ("6.2".equals(bwcBranch)) {
       environment('JAVA_HOME', getJavaHome(it, 9))
-    } else if (["6.3", "6.4", "6.x"].contains(bwcBranch)) {
+    } else if (["6.3", "6.4"].contains(bwcBranch)) {
       environment('JAVA_HOME', getJavaHome(it, 10))
+    } else if (["6.x"].contains(bwcBranch)) {
+      environment('JAVA_HOME', getJavaHome(it, 11))
     } else {
       environment('JAVA_HOME', project.compilerJavaHome)
     }