Add mention of pg_upgrade for release checklist.
Add mention of pg_upgrade for release checklist.

xinzweb/gpdb,randomtask1155/gpdb,yuanzhao/gpdb,edespino/gpdb,Chibin/gpdb,zaksoup/gpdb,lisakowen/gpdb,ahachete/gpdb,50wu/gpdb,oberstet/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,foyzur/gpdb,Quikling/gpdb,tpostgres-projects/tPostgres,zeroae/postgres-xl,kaknikhil/gpdb,cjcjameson/gpdb,50wu/gpdb,0x0FFF/gpdb,yuanzhao/gpdb,50wu/gpdb,oberstet/postgres-xl,janebeckman/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,xuegang/gpdb,zaksoup/gpdb,kaknikhil/gpdb,yuanzhao/gpdb,edespino/gpdb,rubikloud/gpdb,ashwinstar/gpdb,rubikloud/gpdb,arcivanov/postgres-xl,CraigHarris/gpdb,janebeckman/gpdb,janebeckman/gpdb,pavanvd/postgres-xl,yazun/postgres-xl,lpetrov-pivotal/gpdb,lisakowen/gpdb,jmcatamney/gpdb,adam8157/gpdb,snaga/postgres-xl,techdragon/Postgres-XL,Chibin/gpdb,edespino/gpdb,rubikloud/gpdb,cjcjameson/gpdb,tangp3/gpdb,arcivanov/postgres-xl,cjcjameson/gpdb,randomtask1155/gpdb,ashwinstar/gpdb,ahachete/gpdb,rvs/gpdb,pavanvd/postgres-xl,kaknikhil/gpdb,yuanzhao/gpdb,ovr/postgres-xl,xuegang/gpdb,yazun/postgres-xl,lintzc/gpdb,tangp3/gpdb,yazun/postgres-xl,kaknikhil/gpdb,50wu/gpdb,atris/gpdb,greenplum-db/gpdb,ahachete/gpdb,techdragon/Postgres-XL,janebeckman/gpdb,janebeckman/gpdb,edespino/gpdb,randomtask1155/gpdb,lisakowen/gpdb,kaknikhil/gpdb,lisakowen/gpdb,cjcjameson/gpdb,zaksoup/gpdb,kaknikhil/gpdb,tangp3/gpdb,kaknikhil/gpdb,yazun/postgres-xl,Chibin/gpdb,kmjungersen/PostgresXL,greenplum-db/gpdb,lintzc/gpdb,edespino/gpdb,rubikloud/gpdb,royc1/gpdb,jmcatamney/gpdb,tangp3/gpdb,adam8157/gpdb,foyzur/gpdb,xuegang/gpdb,tangp3/gpdb,rvs/gpdb,edespino/gpdb,lintzc/gpdb,randomtask1155/gpdb,0x0FFF/gpdb,greenplum-db/gpdb,atris/gpdb,greenplum-db/gpdb,rvs/gpdb,greenplum-db/gpdb,50wu/gpdb,Quikling/gpdb,royc1/gpdb,Quikling/gpdb,foyzur/gpdb,ovr/postgres-xl,techdragon/Postgres-XL,tpostgres-projects/tPostgres,xuegang/gpdb,kmjungersen/PostgresXL,Quikling/gpdb,Chibin/gpdb,ahachete/gpdb,ahachete/gpdb,Quikling/gpdb,cjcjameson/gpdb,arcivanov/postgres-xl,lpetrov-pivotal/gpdb,atris/gpdb,0x0FFF/gpdb,royc1/gpdb,zaksoup/gpdb,rvs/gpdb,xuegang/gpdb,arcivanov/postgres-xl,Chibin/gpdb,edespino/gpdb,pavanvd/postgres-xl,lintzc/gpdb,atris/gpdb,cjcjameson/gpdb,greenplum-db/gpdb,atris/gpdb,foyzur/gpdb,greenplum-db/gpdb,adam8157/gpdb,tpostgres-projects/tPostgres,jmcatamney/gpdb,rubikloud/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,0x0FFF/gpdb,oberstet/postgres-xl,Quikling/gpdb,cjcjameson/gpdb,rvs/gpdb,yazun/postgres-xl,arcivanov/postgres-xl,royc1/gpdb,Postgres-XL/Postgres-XL,snaga/postgres-xl,foyzur/gpdb,foyzur/gpdb,kmjungersen/PostgresXL,janebeckman/gpdb,royc1/gpdb,lpetrov-pivotal/gpdb,xinzweb/gpdb,janebeckman/gpdb,oberstet/postgres-xl,CraigHarris/gpdb,xuegang/gpdb,jmcatamney/gpdb,lintzc/gpdb,rubikloud/gpdb,Chibin/gpdb,chrishajas/gpdb,zaksoup/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,Chibin/gpdb,kaknikhil/gpdb,ahachete/gpdb,0x0FFF/gpdb,ahachete/gpdb,xuegang/gpdb,randomtask1155/gpdb,lisakowen/gpdb,kmjungersen/PostgresXL,postmind-net/postgres-xl,lpetrov-pivotal/gpdb,adam8157/gpdb,edespino/gpdb,lintzc/gpdb,cjcjameson/gpdb,pavanvd/postgres-xl,Quikling/gpdb,0x0FFF/gpdb,randomtask1155/gpdb,ashwinstar/gpdb,lisakowen/gpdb,tangp3/gpdb,jmcatamney/gpdb,postmind-net/postgres-xl,janebeckman/gpdb,techdragon/Postgres-XL,CraigHarris/gpdb,edespino/gpdb,rvs/gpdb,edespino/gpdb,janebeckman/gpdb,zaksoup/gpdb,lisakowen/gpdb,CraigHarris/gpdb,tpostgres-projects/tPostgres,tpostgres-projects/tPostgres,zeroae/postgres-xl,ovr/postgres-xl,CraigHarris/gpdb,postmind-net/postgres-xl,chrishajas/gpdb,yuanzhao/gpdb,pavanvd/postgres-xl,foyzur/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,lintzc/gpdb,snaga/postgres-xl,chrishajas/gpdb,Quikling/gpdb,lisakowen/gpdb,rubikloud/gpdb,lpetrov-pivotal/gpdb,postmind-net/postgres-xl,CraigHarris/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,arcivanov/postgres-xl,CraigHarris/gpdb,atris/gpdb,chrishajas/gpdb,Postgres-XL/Postgres-XL,ashwinstar/gpdb,zeroae/postgres-xl,greenplum-db/gpdb,ashwinstar/gpdb,50wu/gpdb,atris/gpdb,postmind-net/postgres-xl,oberstet/postgres-xl,kaknikhil/gpdb,zeroae/postgres-xl,chrishajas/gpdb,zaksoup/gpdb,tangp3/gpdb,adam8157/gpdb,jmcatamney/gpdb,zaksoup/gpdb,xinzweb/gpdb,rvs/gpdb,lpetrov-pivotal/gpdb,xinzweb/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,royc1/gpdb,yuanzhao/gpdb,xinzweb/gpdb,ovr/postgres-xl,xuegang/gpdb,lintzc/gpdb,cjcjameson/gpdb,xuegang/gpdb,Chibin/gpdb,Chibin/gpdb,Quikling/gpdb,ahachete/gpdb,randomtask1155/gpdb,janebeckman/gpdb,Chibin/gpdb,tangp3/gpdb,lpetrov-pivotal/gpdb,rvs/gpdb,chrishajas/gpdb,royc1/gpdb,jmcatamney/gpdb,lintzc/gpdb,snaga/postgres-xl,50wu/gpdb,rubikloud/gpdb,royc1/gpdb,0x0FFF/gpdb,Quikling/gpdb,kmjungersen/PostgresXL,adam8157/gpdb,rvs/gpdb,lpetrov-pivotal/gpdb,chrishajas/gpdb,cjcjameson/gpdb,yuanzhao/gpdb,foyzur/gpdb,50wu/gpdb,ashwinstar/gpdb,CraigHarris/gpdb,yuanzhao/gpdb,Postgres-XL/Postgres-XL,randomtask1155/gpdb,0x0FFF/gpdb,CraigHarris/gpdb,adam8157/gpdb,snaga/postgres-xl,atris/gpdb,chrishajas/gpdb,rvs/gpdb