Make the plugin testable on Java 21 and let dependabot handle it in t…

…he future (#281)

.github/dependabot.yml

@@ -0,0 +1,11 @@
+version: 2
+updates:
+  - package-ecosystem: maven
+    directory: "/"
+    schedule:
+      interval: monthly
+    target-branch: master
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: monthly

Jenkinsfile

@@ -1,4 +1,5 @@
 buildPlugin(useContainerAgent: true, configurations: [
   [platform: 'linux', jdk: 17],
+  [platform: 'linux', jdk: 21],
   [platform: 'windows', jdk: 11],
 ])

pom.xml

@@ -29,7 +29,7 @@ THE SOFTWARE.
   <parent>
     <groupId>org.jenkins-ci.plugins</groupId>
     <artifactId>plugin</artifactId>
-    <version>4.66</version>
+    <version>4.73</version>
     <relativePath />
   </parent>