Define and use jenkins.baseline property (#327)

Avoid updates to jenkins.version that do not also update the Jenkins
plugin bill of materials version.

jenkinsci/archetypes#737 is the inspiration.
Thanks @timja!

pom.xml

@@ -51,7 +51,8 @@
   <properties>
     <changelist>-SNAPSHOT</changelist>
     <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>
-    <jenkins.version>2.426.3</jenkins.version>
+    <jenkins.baseline>2.426</jenkins.baseline>
+    <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <revision>1.12.1</revision>
     <spotbugs.effort>Max</spotbugs.effort>
@@ -63,7 +64,7 @@
     <dependencies>
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
-        <artifactId>bom-2.426.x</artifactId>
+        <artifactId>bom-${jenkins.baseline}.x</artifactId>
         <version>3208.vb_21177d4b_cd9</version>
         <type>pom</type>
         <scope>import</scope>