Remove support for centos7 and ubuntu18.04
Remove support for centos7 and ubuntu18.04

Co-authored-by: Lucas Bonner <54393c882c6afc1deae575bec937ac78c1977860@vmware.com>
Co-authored-by: Shaoqi Bai <7e128f5f13b82562f8df0d475a28b2c22e78dec3@vmware.com>

greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb