Require Jenkins 2.528.3 or newer (#658)

Core security advisories have been issued for versions before 2.528.3 and
"Choosing a Jenkins version to build against" recommends 2.528.3 as the
minimum version.

Testing done:

* Confirmed that automated tests pass on Linux with Java 21

Author: MarkEWaite

pom.xml

@@ -35,7 +35,7 @@
     <gitHubRepo>jenkinsci/priority-sorter-plugin</gitHubRepo>
     <hpi.strictBundledArtifacts>true</hpi.strictBundledArtifacts>
     <!-- https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/ -->
-    <jenkins.baseline>2.504</jenkins.baseline>
+    <jenkins.baseline>2.528</jenkins.baseline>
     <jenkins.version>${jenkins.baseline}.3</jenkins.version>
     <spotbugs.effort>Max</spotbugs.effort>
     <spotbugs.threshold>Low</spotbugs.threshold>