[MNG-6399] Lift JDK minimum to JDK 8

[turbanoff]

Update README.txt in 3.9 branch to mention that Java 8 is required.

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[cstamas]

Related issue https://issues.apache.org/jira/browse/MNG-8011

Hence, I'd trim this README even more, not just update. Just place a link to https://maven.apache.org/docs/history.html IMHO