Avoid bom version mismatches with jenkins.baseline (#403)

The Jenkins plugin archetype uses jenkins.baseline to prevent
inconsistencies between the minimum required Jenkins version and the
Jenkins plugin bill of materials version.  Use the same technique in
this plugin.

pom.xml

@@ -33,7 +33,8 @@
   <properties>
     <changelist>999999-SNAPSHOT</changelist>
     <gitHubRepo>jenkinsci/parameterized-trigger-plugin</gitHubRepo>
-    <jenkins.version>2.440.3</jenkins.version>
+    <jenkins.baseline>2.440</jenkins.baseline>
+    <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <spotbugs.effort>Max</spotbugs.effort>
     <spotbugs.threshold>Low</spotbugs.threshold>
     <spotless.check.skip>false</spotless.check.skip>
@@ -43,7 +44,7 @@
     <dependencies>
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
-        <artifactId>bom-2.440.x</artifactId>
+        <artifactId>bom-${jenkins.baseline}.x</artifactId>
         <version>3435.v238d66a_043fb_</version>
         <type>pom</type>
         <scope>import</scope>