Use bundled Java for all eligible versions in tests (elastic#40928)

This commit sets the version to ensure that we use the bundled Java when
running integration tests for all eligible versions. In particular,
since we started bundling Java with 7.0.0, this commits sets said
version to 7.0.0.

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

@@ -663,8 +663,7 @@ 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
-            // TODO: this must change to 7.0.0 after bundling java has been backported
-            if (project.isRuntimeJavaHomeSet || node.nodeVersion.before(Version.fromString("8.0.0")) ||
+            if (project.isRuntimeJavaHomeSet || node.nodeVersion.before(Version.fromString("7.0.0")) ||
                 node.config.distribution == 'integ-test-zip') {
                 exec.environment.put('JAVA_HOME', project.runtimeJavaHome)
             } else {
@@ -690,7 +689,7 @@ class ClusterFormationTasks {
             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.nodeVersion.before(Version.fromString("8.0.0")) ||
+                if (project.isRuntimeJavaHomeSet || node.nodeVersion.before(Version.fromString("7.0.0")) ||
                     node.config.distribution == 'integ-test-zip') {
                     env(key: 'JAVA_HOME', value: project.runtimeJavaHome)
                 }