Release 2.0.0-RC2

Second v2 release candidate.

Main focus of the 'v2' version is simplification of this library.
In the past, many features were added to the library. Some introduced more complexity than technically needed.
For v2 we redesigned some api elements that allow simpler implementation, reducing complexity.

• Java version bumped to Java8 minimum
• ContextManager.getActiveContext() replaced by getActiveContextValue().
• ContextManager.clear() will require an implementation.
• Context is no longer a generic interface.
• All @Deprecated(forRemoval=true) is to be removed.
• Add caveat about use vs. Scoped values and Structured Concurrency.