Upgrade Dockerfile with java 21

Dockerfile

@@ -1,4 +1,4 @@
-FROM openjdk:17-jdk-slim as BuildJava
+FROM openjdk:21-slim as BuildJava
 ARG JAR_FILE=target/stocks-0.1.jar
 COPY ${JAR_FILE} app.jar
 ENTRYPOINT ["java","-jar","/app.jar"]