Require Jenkins 2.440.3 or newer (#325)

The plugin bill of materials for 2.426.x has received its last update.
Switch to Jenkins 2.440.3 as the minimum Jenkins version.

Security fixes in Jenkins 2.440.3 are good for users and most users that
are actively upgrading Jenkins core and plugins are already running a
newer version.

pom.xml

@@ -44,7 +44,7 @@
     <changelist>999999-SNAPSHOT</changelist>
     <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>
     <!-- https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/ -->
-    <jenkins.baseline>2.426</jenkins.baseline>
+    <jenkins.baseline>2.440</jenkins.baseline>
     <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <spotbugs.effort>Max</spotbugs.effort>
     <spotbugs.threshold>Low</spotbugs.threshold>