Require Jenkins 2.440.3 or newer (#130)

https://stats.jenkins.io/pluginversions/cloud-stats.html shows that
Jenkins 2.440.3 or newer is already used by 80% of the installations
of the most recent release and the most recent release is used in 75%
of all installations.  Jenkins 2.440.3 also includes a security fix that
is good for users to deploy.

BOM updates have stopped for bom-2.426.x so this will also continue
updating plugin dependencies by switching to a bom version that is still
receiving updates.

Change also removes the duplication of the Jenkins baseline as has been
done recently in the plugin archetype.  Refer to:

* jenkinsci/archetypes#737

pom.xml

@@ -43,7 +43,8 @@
   <properties>
     <changelist>999999-SNAPSHOT</changelist>
     <gitHubRepo>jenkinsci/cloud-stats-plugin</gitHubRepo>
-    <jenkins.version>2.426.3</jenkins.version>
+    <jenkins.baseline>2.440</jenkins.baseline>
+    <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <spotbugs.effort>Max</spotbugs.effort>
     <spotbugs.threshold>Low</spotbugs.threshold>
     <surefire.useFile>false</surefire.useFile>
@@ -55,8 +56,8 @@
     <dependencies>
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
-        <artifactId>bom-2.426.x</artifactId>
-        <version>3208.vb_21177d4b_cd9</version>
+        <artifactId>bom-${jenkins.baseline}.x</artifactId>
+        <version>3221.ve8f7b_fdd149d</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>