Release Note: Remove support for 1.5 and earlier source and target options

The `javac` command no longer supports `-source` or` -target` values for releases before 6/1.6. However, older class files are still readable by `javac`. Source code for older release can be ported to a newer source level. To generate class files usable by releases older than JDK 6, a `javac` from a JDK 6, 7, or 8 release family can be used.

[JEP 182](http://openjdk.java.net/jeps/182) documents the policy for retiring old `-source` and `-target` options.