Improve support for Java 21 (#1400)

Java 21 was released Sep 19, 2023. We want to announce full support for
Java 21 in early October and would like the most used plugins to be
compiled and tested with Java 21.

The acceptance test harness and plugin bill of materials tests are
already passing with Java 21. This is a further step to improve plugin
readiness for use with Java 21 and for development with Java 21.

The change intentionally tests only two Java configurations, Java 17 and
Java 21 because we believe that the risk of a regression that only
affects Java 11 is shallow. We generate Java 11 byte code with the Java
17 and the Java 21 builds, so we're already testing Java 11 byte code.

Jenkinsfile

@@ -1,4 +1,4 @@
 buildPlugin(useContainerAgent: true, configurations: [
-    [platform: 'linux', jdk: 17],
-    [platform: 'windows', jdk: 11],
+    [platform: 'linux', jdk: 21],
+    [platform: 'windows', jdk: 17],
 ])

pom.xml

@@ -4,7 +4,7 @@
   <parent>
     <groupId>org.jenkins-ci.plugins</groupId>
     <artifactId>plugin</artifactId>
-    <version>4.72</version>
+    <version>4.74</version>
     <relativePath />
   </parent>
   <artifactId>job-dsl-parent</artifactId>