Add TWENTY_ONE to JavaVersion

Closes gh-37362

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/system/JavaVersion.java

@@ -23,6 +23,7 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
+import java.util.SortedSet;
 import java.util.concurrent.Future;
 import java.util.stream.Stream;
 
@@ -114,7 +115,13 @@ public enum JavaVersion {
 	 * Java 20.
 	 * @since 2.7.13
 	 */
-	TWENTY("20", Class.class, "accessFlags");
+	TWENTY("20", Class.class, "accessFlags"),
+
+	/**
+	 * Java 21.
+	 * @since 2.7.16
+	 */
+	TWENTY_ONE("21", SortedSet.class, "getFirst");
 
 	private final String name;
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/system/JavaVersionTests.java

@@ -158,6 +158,12 @@ class JavaVersionTests {
 		assertThat(JavaVersion.getJavaVersion()).isEqualTo(JavaVersion.TWENTY);
 	}
 
+	@Test
+	@EnabledIf("java21")
+	void currentJavaVersionTwentyOne() {
+		assertThat(JavaVersion.getJavaVersion()).isEqualTo(JavaVersion.TWENTY_ONE);
+	}
+
 	static boolean java19() {
 		return "19".equals(System.getProperty("java.version"));
 	}
@@ -166,4 +172,8 @@ class JavaVersionTests {
 		return System.getProperty("java.version").startsWith("20");
 	}
 
+	static boolean java21() {
+		return System.getProperty("java.version").startsWith("21");
+	}
+
 }