POM: bump to pom-imagej 2.7

This version is available from Central!
POM: remove hardcoded ImageJ repository

It is no longer needed since the parent POM (pom-imagej version 2.7)
is now available from Maven Central.
POM: tidy up order of configuration blocks

This was done using:
   mvn tidy:pom
POM: add missing issueManagement and ciManagement