docs - add gpcopy as an option to migrate data to GPDB 6 (#8751)
docs - add gpcopy as an option to migrate data to GPDB 6 (#8751)

* Can use gpcopy to migrate data

* Conditionalize gpcopy references

* Edits for review comments

lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb