Update CHANGES and UPGRADE.txt for 20.0.0

doc/CHANGES-staging/func_channel.txt

doc/CHANGES-staging/func_db.txt

doc/CHANGES-staging/func_env.txt

doc/CHANGES-staging/func_evalexten.txt

doc/CHANGES-staging/func_framedrop.txt

doc/CHANGES-staging/func_json.txt

doc/CHANGES-staging/func_odbc_esc_backslashes.txt

doc/CHANGES-staging/func_scramble.txt

doc/CHANGES-staging/func_strings.txt

doc/CHANGES-staging/func_vmcount.txt

doc/CHANGES-staging/load_realtime_queues.txt

doc/CHANGES-staging/logger.txt

doc/CHANGES-staging/manager_disable.txt

doc/CHANGES-staging/manager_message_send.txt

doc/CHANGES-staging/mf.txt

doc/CHANGES-staging/pjsip_read_headers.txt

doc/CHANGES-staging/queue_withdraw_caller.txt

doc/CHANGES-staging/res_agi.txt

doc/CHANGES-staging/res_cliexec.txt

doc/CHANGES-staging/res_fax_spandsp.txt