Add 13, force 9/10 -> 11

• 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