Bump docker-workflow from 1.18 to 1.21

Bumps [docker-workflow](https://github.com/jenkinsci/docker-workflow-plugin) from 1.18 to 1.21.
- [Release notes](https://github.com/jenkinsci/docker-workflow-plugin/releases)
- [Commits](jenkinsci/docker-workflow-plugin@docker-workflow-1.18...docker-workflow-1.21)

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

pom.xml

@@ -221,7 +221,7 @@
     <dependency>
       <groupId>org.jenkins-ci.plugins</groupId>
       <artifactId>docker-workflow</artifactId>
-      <version>1.18</version>
+      <version>1.21</version>
       <scope>test</scope>
     </dependency>
     <dependency>