Bump org.jenkins-ci.plugins:timestamper from 1.27 to 1.28

Bumps [org.jenkins-ci.plugins:timestamper](https://github.com/jenkinsci/timestamper-plugin) from 1.27 to 1.28.
- [Release notes](https://github.com/jenkinsci/timestamper-plugin/releases)
- [Changelog](https://github.com/jenkinsci/timestamper-plugin/blob/master/CHANGELOG.adoc)
- [Commits](jenkinsci/timestamper-plugin@timestamper-1.27...timestamper-1.28)

---
updated-dependencies:
- dependency-name: org.jenkins-ci.plugins:timestamper
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

plugin/pom.xml

@@ -263,7 +263,7 @@
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>timestamper</artifactId>
-      <version>1.27</version>
+      <version>1.28</version>
       <scope>test</scope>
     </dependency>