Update parent POM, update BOM, test with Java 21 (#261)

* Bump org.jenkins-ci.plugins:plugin from 4.71 to 4.74

Bumps [org.jenkins-ci.plugins:plugin](https://github.com/jenkinsci/plugin-pom) from 4.71 to 4.74.
- [Release notes](https://github.com/jenkinsci/plugin-pom/releases)
- [Changelog](https://github.com/jenkinsci/plugin-pom/blob/master/CHANGELOG.md)
- [Commits](jenkinsci/plugin-pom@plugin-4.71...plugin-4.74)

---
updated-dependencies:
- dependency-name: org.jenkins-ci.plugins:plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

* Bump io.jenkins.tools.bom:bom-2.387.x

Bumps [io.jenkins.tools.bom:bom-2.387.x](https://github.com/jenkinsci/bom) from 2278.v47b_4508e256a to 2496.vddfca_753db_80.
- [Release notes](https://github.com/jenkinsci/bom/releases)
- [Commits](https://github.com/jenkinsci/bom/commits)

---
updated-dependencies:
- dependency-name: io.jenkins.tools.bom:bom-2.387.x
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

* Test with JDK 21.

* Apply suggestions from Mark's code review

Co-authored-by: Mark Waite <[email protected]>

---------

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Mark Waite <[email protected]>

Author: 3 people

Jenkinsfile

@@ -5,6 +5,6 @@
 buildPlugin(
   useContainerAgent: true, // Set to `false` if you need to use Docker for containerized tests
   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>4.71</version>
+    <version>4.76</version>
     <relativePath />
   </parent>
 
@@ -100,7 +100,7 @@
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
         <artifactId>bom-2.387.x</artifactId>
-        <version>2278.v47b_4508e256a</version>
+        <version>2543.vfb_1a_5fb_9496d</version>
         <scope>import</scope>
         <type>pom</type>
       </dependency>