Update CHANGES and UPGRADE.txt for 18.5.0

CHANGES

UPGRADE.txt

doc/CHANGES-staging/app_confbridge_answer.txt

doc/CHANGES-staging/app_confkick.txt

doc/CHANGES-staging/app_originate_vars.txt

doc/CHANGES-staging/flash_ami_event.txt

doc/CHANGES-staging/func_min_max.txt

doc/CHANGES-staging/func_volume_read.txt

doc/CHANGES-staging/messagesend.txt

doc/CHANGES-staging/res_pjsip.txt

doc/CHANGES-staging/res_pjsip_dialog_info_body_generator.txt

doc/CHANGES-staging/res_pjsip_dtmf.txt

doc/CHANGES-staging/res_pjsip_messaging.txt

...NGES-staging/res_rtp_asterisk_stun_software_attribute.txt

doc/CHANGES-staging/voicemail_beep.txt

doc/UPGRADE-staging/stir-shaken-public-key-url.txt

doc/UPGRADE-staging/stir_shaken_origid.txt