Use jenkins.baseline to match archetype (#169)

* Use dependabot config from archetype

* Avoid bom version mismatches with jenkins.baseline

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.

.github/dependabot.yml

@@ -1,10 +1,12 @@
-# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuring-dependabot-version-updates
----
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuring-dependabot-version-updates
+
 version: 2
 updates:
-  - package-ecosystem: "maven"
-    directory: "/"
-    labels:
-      - "dependencies"
-    schedule:
-      interval: "monthly"
+- package-ecosystem: maven
+  directory: /
+  schedule:
+    interval: monthly
+- package-ecosystem: github-actions
+  directory: /
+  schedule:
+    interval: monthly

pom.xml

@@ -31,7 +31,8 @@
     </scm>
 
     <properties>
-        <jenkins.version>2.426.3</jenkins.version>
+        <jenkins.baseline>2.426</jenkins.baseline>
+        <jenkins.version>${jenkins.baseline}.3</jenkins.version>
         <revision>1.34</revision>
         <changelist>-SNAPSHOT</changelist>
         <spotbugs.effort>Max</spotbugs.effort>
@@ -42,7 +43,7 @@
         <dependencies>
             <dependency>
                 <groupId>io.jenkins.tools.bom</groupId>
-                <artifactId>bom-2.426.x</artifactId>
+                <artifactId>bom-${jenkins.baseline}.x</artifactId>
                 <version>3208.vb_21177d4b_cd9</version>
                 <type>pom</type>
                 <scope>import</scope>