Switching to Java 21

Author: WadeWaldron

staging/04-connecting-to-confluent-cloud/pom.xml

@@ -31,7 +31,7 @@ under the License.
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <flink.version>1.20-SNAPSHOT</flink.version>
         <confluent-plugin.version>0.44.0</confluent-plugin.version>
-        <target.java.version>17</target.java.version>
+        <target.java.version>21</target.java.version>
         <maven.compiler.source>${target.java.version}</maven.compiler.source>
         <maven.compiler.target>${target.java.version}</maven.compiler.target>
         <log4j.version>2.17.1</log4j.version>