Bump org.codehaus.plexus:plexus from 20 to 21

Bumps [org.codehaus.plexus:plexus](https://github.com/codehaus-plexus/plexus-pom) from 20 to 21.
- [Release notes](https://github.com/codehaus-plexus/plexus-pom/releases)
- [Commits](https://github.com/codehaus-plexus/plexus-pom/commits)

---
updated-dependencies:
- dependency-name: org.codehaus.plexus:plexus
  dependency-version: '21'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pom.xml

@@ -20,7 +20,7 @@ limitations under the License.
   <parent>
     <groupId>org.codehaus.plexus</groupId>
     <artifactId>plexus</artifactId>
-    <version>20</version>
+    <version>21</version>
   </parent>
 
   <artifactId>plexus-utils</artifactId>