* Remove comment references to 9 and 10 as possible version specifications.
Note that 9 and 10 aren't actually removed as valid versions as there is
one port with a reference (news/nzbhydra2).
* Add 13 as a valid version.
* Add configuration for openjdk13.
* Modify the construction of _JAVA_VERSION to force 9 and 10 to 11.
Sponsored by: The FreeBSD Foundation