Fixing the --release flag usage for javac (#2343)

* Fixing the --release flag usage for javac

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>

* Fixing the --html5 flag usage for javadoc

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>

buildSrc/src/main/java/org/opensearch/gradle/OpenSearchJavaPlugin.java

@@ -173,7 +173,10 @@ public static void configureCompile(Project project) {
                 // workaround for https://github.com/gradle/gradle/issues/14141
                 compileTask.getConventionMapping().map("sourceCompatibility", () -> java.getSourceCompatibility().toString());
                 compileTask.getConventionMapping().map("targetCompatibility", () -> java.getTargetCompatibility().toString());
-                compileOptions.getRelease().set(releaseVersionProviderFromCompileTask(project, compileTask));
+                // The '--release is available from JDK-9 and above
+                if (BuildParams.getRuntimeJavaVersion().compareTo(JavaVersion.VERSION_1_8) > 0) {
+                    compileOptions.getRelease().set(releaseVersionProviderFromCompileTask(project, compileTask));
+                }
             });
             // also apply release flag to groovy, which is used in build-tools
             project.getTasks().withType(GroovyCompile.class).configureEach(compileTask -> {
@@ -267,7 +270,9 @@ private static void configureJavadoc(Project project) {
              * that the default will change to html5 in the future.
              */
             CoreJavadocOptions javadocOptions = (CoreJavadocOptions) javadoc.getOptions();
-            javadocOptions.addBooleanOption("html5", true);
+            if (BuildParams.getRuntimeJavaVersion().compareTo(JavaVersion.VERSION_1_8) > 0) {
+                javadocOptions.addBooleanOption("html5", true);
+            }
         });
 
         TaskProvider<Javadoc> javadoc = project.getTasks().withType(Javadoc.class).named("javadoc");