[MRELEASE-1130] Reject release of project containing undefined plugin versions

**[Benjamin Marwell](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=mampf86)** opened **[MRELEASE-1130](https://issues.apache.org/jira/browse/MRELEASE-1130?redirect=false)** and commented

As discussed in slack, the m-release-p should reject releases where at least one plugin version was not set.

The warning didn't cut it and can be removed once this is done, because it does not really matter for local builds – unless a dev uses an old maven version which leads to a bug, but then there is Maven Wrapper to rule that out.

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**Issue Links:**
- [MNG-7808](https://issues.apache.org/jira/browse/MNG-7808) Remove warnings of undefined plugin versions
(_**"is required by"**_)

Start by tracing the m-release-p release validation for plugins whose versions are undefined, then review the existing warning referenced in the issue. Check the linked MNG-7808 context before changing behavior; done means such a release is rejected and the obsolete warning can be removed without affecting local builds.

Written by the indexing model from the issue text.