Stop testing Java 11

Jenkins stopped supporting Java 11 with the release of Jenkins 2.463
(weekly) and Jenkins 2.479.1 (LTS).  Most plugins stopped spending
ci.jenkins.io time to run tests that are specific to Java 11.

Let's save money and time by removing the Java 11 test configuration.
It has not found any issues that are not also found with Java 17 and
Java 21.

Jenkins plugin BOM still tests with Java 11 on older lines (currently
2.452.x and 2.462.x) and the plugin build will continue to generate Java
11 byte code until the parent pom is upgraded to 5.x and the minimum
Jenkins version is upgraded to 2.479.1.

Author: MarkEWaite

Jenkinsfile

@@ -1,7 +1,5 @@
 // Builds the plugin using https://github.com/jenkins-infra/pipeline-library
 buildPlugin(useContainerAgent: true, configurations: [
-  // Test the minimum required Jenkins Version.
-  [ platform: 'linux', jdk: '11'],
   [ platform: 'windows', jdk: '17'],
   [ platform: 'linux', jdk: '21']
 ])