Define and use jenkins.baseline property (#215)

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

@@ -44,7 +44,8 @@
     <changelist>999999-SNAPSHOT</changelist>
     <gitHubRepo>jenkinsci/implied-labels-plugin</gitHubRepo>
     <!-- https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/ -->
-    <jenkins.version>2.426.3</jenkins.version>
+    <jenkins.baseline>2.426</jenkins.baseline>
+    <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <spotbugs.effort>Max</spotbugs.effort>
     <spotbugs.threshold>Low</spotbugs.threshold>
     <spotless.check.skip>false</spotless.check.skip>
@@ -54,7 +55,7 @@
     <dependencies>
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
-        <artifactId>bom-2.426.x</artifactId>
+        <artifactId>bom-${jenkins.baseline}.x</artifactId>
         <version>3208.vb_21177d4b_cd9</version>
         <type>pom</type>
         <scope>import</scope>