Support for Java 6 is being completely dropped according to the Jenkins CI blog. Additionally, Maven 3.3+ only supports Java 1.7+.

Maven 3.3 requires JDK 1.7 or above to execute - it still allows you to build against 1.3 and other JDK versions by Using Toolchains

Therefore, it is imperative that the GitHub OAuth plugin be ported to Java 7 and even Java 8. Otherwise, it will likely not be maintainable as time progresses.