Make the Cookie's SameSite directive property string value final

See spring-projects#15047

Author: cleankod

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/server/Session.java

@@ -244,7 +244,7 @@ public enum SameSite {
 			 */
 			NONE("None");
 
-			private String value;
+			private final String value;
 
 			SameSite(final String value) {
 				this.value = value;