[MNG-6667] Hint at Maven upgrade requirement when trying to build a pom.xml with a newer modelVersion

[Stephen Connolly]

Finally got some time to do some work on Maven again... who knows how long
it will last!!!

https://issues.apache.org/jira/browse/MNG-6667

https://gitbox.apache.org/repos/asf?p=maven.git;a=commit;h=7376a99093984c459f6a70cd1f508bbcf5ef26f7

WDYT?

If CI passes are we good to merge?

-Stephen