Bump plexus from 7 to 8 (#179)

Bumps [plexus](https://github.com/codehaus-plexus/plexus-pom) from 7 to 8.
- [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-type: direct:production
  update-type: version-update:semver-major
...

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pom.xml

@@ -6,7 +6,7 @@
   <parent>
     <groupId>org.codehaus.plexus</groupId>
     <artifactId>plexus</artifactId>
-    <version>7</version>
+    <version>8</version>
   </parent>
 
   <artifactId>plexus-archiver</artifactId>