[MNG-7851] Improve error message when modelVersion is 4.0

[gnodet]

Improve DefaultModelValidator.compareModelVersions to be able to compare
versions which different numbers of segments, allowing it to compare
"4", "4.0", and "4.0.0" to each other for example.

Signed-off-by: Craig Andrews candrews@integralblue.com

Conflicts:

maven-model-builder/src/test/java/org/apache/maven/model/validation/DefaultModelValidatorTest.java

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