FYI: Merged genshen into master in openjdk/shenandoah repository

[Roman Kennke]

We decided to simplify things a little bit in Shenandoah repository: The 
work in genshen branch is now merged into master, and brought up-to-date 
with recent JDK17 development. The genshen branch should not be used 
anymore, and any new PRs for the generational Shenandoah work should not 
be made directly against master in openjdk/shenandoah.

Roman