[pom] Bump oss-parent to 9 - the last to ever be released

As noted, the contents should be brought into project as sonatype does not promote this any more but at least be up to date for now.

pom.xml

@@ -23,7 +23,7 @@
     <parent>
         <groupId>org.sonatype.oss</groupId>
         <artifactId>oss-parent</artifactId>
-        <version>7</version>
+        <version>9</version>
         <relativePath />
     </parent>