Update Java Virtual Machine Guide about CDS support for ZGC

Since JDK-8232069 (JDK 15), CDS now supports ZGC. The following part of the Java Virtual Machine Guide needs to be updated

https://docs.oracle.com/en/java/javase/16/vm/class-data-sharing.html#GUID-EC975B2E-B4AB-45B4-B91F-51C3A264D0CE

The current text is:

"Class data sharing is supported with the G1, serial, parallel, and parallelOldGC garbage collectors. The shared Java heap object feature (part of class data sharing) supports only the G1 garbage collector on 64-bit non-Windows platforms."

ZGC should be added to the list. parallelOldGC should be removed since it's no longer supported since JDK 15.