Remove dead code related to java compatibility now that Java min version is 21 (opensearch-project#20143)

Signed-off-by: Craig Perkins <cwperx@amazon.com>

Author: cwperks

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

@@ -174,10 +174,7 @@ 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());
-                // The '--release is available from JDK-9 and above
-                if (BuildParams.getRuntimeJavaVersion().compareTo(JavaVersion.VERSION_1_8) > 0) {
-                    compileOptions.getRelease().set(releaseVersionProviderFromCompileTask(project, compileTask));
-                }
+                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 -> {
@@ -271,9 +268,7 @@ private static void configureJavadoc(Project project) {
              * that the default will change to html5 in the future.
              */
             CoreJavadocOptions javadocOptions = (CoreJavadocOptions) javadoc.getOptions();
-            if (BuildParams.getRuntimeJavaVersion().compareTo(JavaVersion.VERSION_1_8) > 0) {
-                javadocOptions.addBooleanOption("html5", true);
-            }
+            javadocOptions.addBooleanOption("html5", true);
         });
 
         TaskProvider<Javadoc> javadoc = project.getTasks().withType(Javadoc.class).named("javadoc");

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

@@ -40,7 +40,6 @@
 import org.opensearch.gradle.test.ErrorReportingTestListener;
 import org.opensearch.gradle.util.Util;
 import org.gradle.api.Action;
-import org.gradle.api.JavaVersion;
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
 import org.gradle.api.Task;
@@ -106,15 +105,7 @@ public void execute(Task t) {
                     mkdirs(test.getWorkingDir());
                     mkdirs(test.getWorkingDir().toPath().resolve("temp").toFile());
 
-                    // TODO remove once jvm.options are added to test system properties
-                    if (BuildParams.getRuntimeJavaVersion() == JavaVersion.VERSION_1_8) {
-                        test.systemProperty("java.locale.providers", "SPI,JRE");
-                    } else {
-                        test.systemProperty("java.locale.providers", "SPI,CLDR");
-                        if (test.getJavaVersion().compareTo(JavaVersion.VERSION_17) < 0) {
-                            test.jvmArgs("--illegal-access=warn");
-                        }
-                    }
+                    test.systemProperty("java.locale.providers", "SPI,CLDR");
                 }
             });
             test.getJvmArgumentProviders().add(nonInputProperties);