Lower Java version requirement from 11 to 8

[dabico]

Original PR text:

According to a recent article, at least one-third of the Java ecosystem is still comprised of Java 8 applications. The primary choice behind using Java 11 for this library was because of the java.lang.ref.Cleaner API, which was introduced in Java 9. This API provides a memory safety net, in cases where the user forgets to appropriately clean up the native resources associated with External instances.

Although migrating to Java 8 would no doubt contribute to the overall adoption of the library, the removal of the Cleaner API is out of the question. We therefore need to devise alternative means by which we can guarantee native resource cleanup.