Qualifies that KeyFactory and Propagation.Factory.create will go in Brave 7

[codefromthecrypt]

It only takes 2 major versions to safely transition from abstract -> deprecated -> removal

• v5.18 transitions Propagation.Factory.create() from abstract to deprecated with an exception, enforcing Propagation.Factory.get() must be implemented.
• users can notice this on v5.18, but have v6 to make sure libraries don't lock up
• v7 can safely remove Propagation.Factory.create() and Propagation.KeyFactory only called by that.
  • unlikely to have a v7 for a long time, possibly only to address Java version lockup once JDK 11 becomes non LTS