Review request for JMC-6468: Rule for detecting unsupported JDK version

[Marcus Hirt]

Hi all,

Please review this enhancement for warning when an unsupported JDK version 
is being used (such as when an old non-LTS release is in use, or a very 
old no longer supported version, or an early access release).

There are various ways this could be done, but the rule will currently 
be fine with actively supported releases (JDK 8, JDK 11), or the latest 
non-LTS release (currently JDK 12). It will be a little bit grumpy about 
JDK 7 (still supported by some vendors), and decidedly unhappy about 
earlier releases and non-LTS releases not being the latest.

Note that given the update cycle of this rule, and the multitude of vendors
releasing and supporting OpenJDK based JVMs, this rule does not attempt to 
discover if an old interim/update of a supported release is in use. It is 
only complaining about the feature release in use.

See also: http://openjdk.java.net/jeps/322

Jira:   https://bugs.openjdk.java.net/browse/JMC-6468
Webrev: http://cr.openjdk.java.net/~hirt/JMC-6468/webrev.2/

Kind regards,
Marcus