LOG4J2-2993 Support stack trace truncation in JsonTemplateLayout

Hello, I have cloned Ron's LOG4J2-2986 (truncation of stack traces after a matching point) for JSON template layout in LOG4J2-2993 and created a PR

JDK 16 is now in Rampdown Phase Two

Hi Ralph, *Per the JDK 16 schedule , we are in Rampdown Phase Two* *[1] . * *Please advise if you find any issues while testing the latest Early Access builds.* * Schedule for JDK 16 o *2021/01/14  Rampdown Phase Two* o 2021/02/04  Initial Release Candidate o 2021/02/18  Final