Bump configuration-as-code from 1.6 to 1.21

Bumps [configuration-as-code](https://github.com/jenkinsci/configuration-as-code-plugin) from 1.6 to 1.21.
- [Release notes](https://github.com/jenkinsci/configuration-as-code-plugin/releases)
- [Changelog](https://github.com/jenkinsci/configuration-as-code-plugin/blob/master/CHANGELOG.md)
- [Commits](jenkinsci/configuration-as-code-plugin@configuration-as-code-1.6...configuration-as-code-1.21)

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

pom.xml

@@ -69,7 +69,7 @@
         <dependency>
             <groupId>io.jenkins</groupId>
             <artifactId>configuration-as-code</artifactId>
-            <version>1.6</version>
+            <version>1.21</version>
             <scope>test</scope>
         </dependency>
     </dependencies>