Use jenkins.baseline to reduce BOM update mistakes (#2593)

Author: strangelookingnerd

pom.xml

@@ -32,7 +32,9 @@
         when changing jenkins core version please remember to change it in Jenkinsfile as well jenkinsVersions
         and  acceptance-tests/runner/scripts/args.sh
     -->
-    <jenkins.version>2.426.3</jenkins.version>
+    <!-- https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/ -->
+    <jenkins.baseline>2.426</jenkins.baseline>
+    <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <javadoc.exec.goal>javadoc-no-fork</javadoc.exec.goal> <!-- stop initialize phase plugins executing twice -->
     <byte-buddy.version>1.14.9</byte-buddy.version>
     <node.version>10.13.0</node.version>
@@ -219,7 +221,7 @@
 
         <dependency>
             <groupId>io.jenkins.tools.bom</groupId>
-            <artifactId>bom-2.426.x</artifactId>
+            <artifactId>bom-${jenkins.baseline}.x</artifactId>
             <version>2961.v1f472390972e</version>
             <scope>import</scope>
             <type>pom</type>