This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
If you check the download page for JDK 5, you will see that the full name is "Java 2 Platform Standard Edition 5.0". So it is still a release for the Java 2 platform and there is no problem.
In fact, the instructions state that you must use a version of the JDK that has "not have been superseded by a new production version for more than 18 months by the time you make your submission". The only JDK that meets that criteria at present is JDK 5, so you do not really have a choice: you must use JDK 5.