Bump plugin from 3.5 to 4.15

Bumps [plugin](https://github.com/jenkinsci/plugin-pom) from 3.5 to 4.15.
- [Release notes](https://github.com/jenkinsci/plugin-pom/releases)
- [Changelog](https://github.com/jenkinsci/plugin-pom/blob/master/CHANGELOG.md)
- [Commits](jenkinsci/plugin-pom@plugin-3.5...plugin-4.15)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

pom.xml

@@ -4,7 +4,7 @@
     <parent>
         <groupId>org.jenkins-ci.plugins</groupId>
         <artifactId>plugin</artifactId>
-        <version>3.5</version>
+        <version>4.15</version>
     </parent>
     <artifactId>validating-string-parameter</artifactId>
     <packaging>hpi</packaging>