Keep Java version 21 while updating Spring Boot to 3.4.5

Co-Authored-By: Philip Riecks <mail@philipriecks.de>

Author: devin-ai-integration[bot]

README.md

@@ -3,7 +3,7 @@
 [![Masterclass](https://rieckpil.de/wp-content/uploads/2020/09/testing-spring-boot-applications-masterclass-course-logo.png)](https://rieckpil.de/testing-spring-boot-applications-masterclass/)
 
 [![](https://img.shields.io/badge/Spring%20Boot%20Version-3.4.5-orange)](/pom.xml)
-[![](https://img.shields.io/badge/Java%20Version-17-orange)](/pom.xml)
+[![](https://img.shields.io/badge/Java%20Version-21-orange)](/pom.xml)
 [![](https://img.shields.io/badge/Enroll-Now-orange)](https://rieckpil.de/testing-spring-boot-applications-masterclass/)
 
 ## 😓 Tired of the Friday Afternoon Dread?

pom.xml

@@ -17,7 +17,7 @@
   <description>Testing Spring Boot Applications Masterclass</description>
 
   <properties>
-    <java.version>17</java.version>
+    <java.version>21</java.version>
     <node.version>v18.16.0</node.version>
     <spring-cloud-aws.version>3.3.0</spring-cloud-aws.version>
     <testcontainers.version>1.20.6</testcontainers.version>