Update dependency io.jenkins.tools.bom:bom-2.528.x to v6421 (jenkins-infra#1753)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

plugin-modernizer-core/pom.xml

@@ -16,7 +16,7 @@
       <dependency>
         <groupId>io.jenkins.tools.bom</groupId>
         <artifactId>bom-2.528.x</artifactId>
-        <version>6398.v1d26a_dd495e2</version>
+        <version>6421.v4a_efb_4b_3a_61d</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>