Define and use jenkins.baseline property (#212)

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

@@ -45,7 +45,8 @@
     <revision>4.5.14</revision>
     <changelist>999999-SNAPSHOT</changelist>
     <gitHubRepo>jenkinsci/${project.artifactId}-plugin</gitHubRepo>
-    <jenkins.version>2.401.3</jenkins.version>
+    <jenkins.baseline>2.401</jenkins.baseline>
+    <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <httpcore.version>4.4.16</httpcore.version>
     <httpclient.version>${revision}</httpclient.version>
     <httpasyncclient.version>4.1.5</httpasyncclient.version>
@@ -58,7 +59,7 @@
     <dependencies>
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
-        <artifactId>bom-2.401.x</artifactId>
+        <artifactId>bom-${jenkins.baseline}.x</artifactId>
         <version>2745.vc7b_fe4c876fa_</version>
         <type>pom</type>
         <scope>import</scope>