Use parent pom 5.16 (#2677)

The plugin bill of materials fails to run tests with parent
pom versions 5.13 through 5.15.  Details are available in
jenkinsci/bom#5000 (comment) and
in the related pull requests:

* jenkinsci/maven-hpi-plugin#741
* jenkinsci/plugin-pom#1138

Author: MarkEWaite

pom.xml

@@ -4,7 +4,7 @@
   <parent>
     <groupId>org.jenkins-ci.plugins</groupId>
     <artifactId>plugin</artifactId>
-    <version>5.14</version>
+    <version>5.16</version>
     <relativePath />
   </parent>