Test with Java 21 (#319)

* Use parent pom 4.74 and latest bom

* Test Java 21 and Java 17

* Use bom-2.387.x 2543.vfb_1a_5fb_9496d

The bom-2.387.x line is no longer being updated.  Let's use the most
recent version of that bom so that the latest set of dependency
updates are included for that line.

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

@@ -30,7 +30,7 @@ THE SOFTWARE.
   <parent>
     <groupId>org.jenkins-ci.plugins</groupId>
     <artifactId>plugin</artifactId>
-    <version>4.67</version>
+    <version>4.74</version>
     <relativePath />
   </parent>
 
@@ -77,7 +77,7 @@ THE SOFTWARE.
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
         <artifactId>bom-2.387.x</artifactId>
-        <version>2163.v2d916d90c305</version>
+        <version>2543.vfb_1a_5fb_9496d</version>
         <scope>import</scope>
         <type>pom</type>
       </dependency>