Ensure javaVer is prefixed and compared correctly

Author: sjvanrossum

runners/google-cloud-dataflow-java/build.gradle

@@ -308,10 +308,13 @@ tasks.register('examplesJavaRunnerV2IntegrationTestDistroless', Test.class) {
 tasks.register('buildAndPushDistrolessContainerImage', Task.class) {
   // Only Java 17 and 21 are supported.
   // See https://github.com/GoogleContainerTools/distroless/tree/main/java#image-contents.
+  def javaVer = getSupportedJavaVersion()
+  if(project.hasProperty('testJavaVersion')) {
+    javaVer = "java${project.getProperty('testJavaVersion')}"
+  }
   def allowed = ["java17", "java21"]
   doLast {
-    def javaVer = project.findProperty('testJavaVersion')
-    if (!allowed.contains(javaVer)) {
+  if (!allowed.contains(javaVer)) {
       throw new GradleException("testJavaVersion must be one of ${allowed}, got: ${javaVer}")
     }
     if (!project.hasProperty('dockerTag')) {