Bump org.apache.maven:maven-core in /tools/osgicheck-maven-plugin (#287)

Bumps [org.apache.maven:maven-core](https://github.com/apache/maven) from 3.5.0 to 3.8.1.
- [Release notes](https://github.com/apache/maven/releases)
- [Commits](apache/maven@maven-3.5.0...maven-3.8.1)

---
updated-dependencies:
- dependency-name: org.apache.maven:maven-core
  dependency-type: direct:production
...

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tools/osgicheck-maven-plugin/pom.xml

@@ -32,7 +32,7 @@
     </description>
 
     <properties>
-        <maven.version>3.5.0</maven.version>
+        <maven.version>3.8.1</maven.version>
         <felix.java.version>8</felix.java.version>
     </properties>