Adapt release flag to new CLI arguments

The JDK project is in the process of modifying the command-line flags
for various JDK tools (http://openjdk.java.net/jeps/293). In particular,
the release flag on javac has changed from -release to --release. This
commit adapts the build process to this change.

Relates #20420

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -405,9 +405,9 @@ class BuildPlugin implements Plugin<Project> {
                 //options.incremental = true
 
                 if (project.javaVersion == JavaVersion.VERSION_1_9) {
-                    // hack until gradle supports java 9's new "-release" arg
+                    // hack until gradle supports java 9's new "--release" arg
                     assert minimumJava == JavaVersion.VERSION_1_8
-                    options.compilerArgs << '-release' << '8'
+                    options.compilerArgs << '--release' << '8'
                     project.sourceCompatibility = null
                     project.targetCompatibility = null
                 }