Bump pipeline-model-definition from 1.2 to 1.8.4

Bumps [pipeline-model-definition](https://github.com/jenkinsci/pipeline-model-definition-plugin) from 1.2 to 1.8.4.
- [Release notes](https://github.com/jenkinsci/pipeline-model-definition-plugin/releases)
- [Changelog](https://github.com/jenkinsci/pipeline-model-definition-plugin/blob/master/CHANGELOG.md)
- [Commits](jenkinsci/pipeline-model-definition-plugin@pipeline-model-definition-1.2...pipeline-model-definition-1.8.4)

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

plugin/pom.xml

@@ -185,7 +185,7 @@
     <dependency>
       <groupId>org.jenkinsci.plugins</groupId>
       <artifactId>pipeline-model-definition</artifactId>
-      <version>1.2</version>
+      <version>1.8.4</version>
       <scope>test</scope>
     </dependency>