Fix the JVM we use for bwc nodes

Before this change we would run bwc nodes with their bundled jdk if
these supported it, so the passed in runtime JDK was not honored.
This became obvius when running with FIPS.

Closes elastic#41721

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterFormationTasks.groovy

@@ -659,8 +659,7 @@ class ClusterFormationTasks {
     static Task configureExecTask(String name, Project project, Task setup, NodeInfo node, Object[] execArgs) {
         return project.tasks.create(name: name, type: LoggedExec, dependsOn: setup) { Exec exec ->
             exec.workingDir node.cwd
-            if ((project.isRuntimeJavaHomeSet && node.isBwcNode == false) // runtime Java might not be compatible with old nodes
-                    || node.config.distribution == 'integ-test-zip') {
+            if (useRuntimeJava(project, node)) {
                 exec.environment.put('JAVA_HOME', project.runtimeJavaHome)
             } else {
                 // force JAVA_HOME to *not* be set
@@ -678,15 +677,20 @@ class ClusterFormationTasks {
         }
     }
 
+    public static boolean useRuntimeJava(Project project, NodeInfo node) {
+        return (project.isRuntimeJavaHomeSet ||
+                (node.isBwcNode == false && node.nodeVersion.before(Version.fromString("7.0.0"))) ||
+                node.config.distribution == 'integ-test-zip')
+    }
+
     /** Adds a task to start an elasticsearch node with the given configuration */
     static Task configureStartTask(String name, Project project, Task setup, NodeInfo node) {
         // this closure is converted into ant nodes by groovy's AntBuilder
         Closure antRunner = { AntBuilder ant ->
             ant.exec(executable: node.executable, spawn: node.config.daemonize, newenvironment: true,
                      dir: node.cwd, taskname: 'elasticsearch') {
                 node.env.each { key, value -> env(key: key, value: value) }
-                if ((project.isRuntimeJavaHomeSet && node.isBwcNode == false) // runtime Java might not be compatible with old nodes
-                        || node.config.distribution == 'integ-test-zip') {
+                if (useRuntimeJava(project, node)) {
                     env(key: 'JAVA_HOME', value: project.runtimeJavaHome)
                 }
                 node.args.each { arg(value: it) }