Update dependency org.jenkins-ci.plugins:plugin to v5.21 (#290)

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

Author: renovate[bot]

pom.xml

@@ -5,7 +5,7 @@
   <parent>
     <groupId>org.jenkins-ci.plugins</groupId>
     <artifactId>plugin</artifactId>
-    <version>5.20</version>
+    <version>5.21</version>
   </parent>
 
   <groupId>org.jenkins-ci.plugins</groupId>