Update CHANGES and UPGRADE.txt for 18.0.0

CHANGES

UPGRADE.txt

doc/CHANGES-staging/ACN_streams.txt

doc/CHANGES-staging/ARI.txt

doc/CHANGES-staging/ami_sendtext_content_type.txt

...HANGES-staging/app_bridgeaddchan_add_bridgeresult_var.txt

doc/CHANGES-staging/app_chanisavail_empty_device_list.txt

doc/CHANGES-staging/app_confbridge_maximum_sample_rate.txt

doc/CHANGES-staging/app_dial_empty_dial_list.txt

doc/CHANGES-staging/app_mixmonitor_wav49.txt

doc/CHANGES-staging/app_page_empty_page_list.txt

doc/CHANGES-staging/ari-bridges-inhibit-colp.txt

doc/CHANGES-staging/ari_create_with_variables.txt

doc/CHANGES-staging/chan_pjsip_moh_passthrough.txt

doc/CHANGES-staging/chan_rtp.txt

doc/CHANGES-staging/confbridge_text_messaging.txt

doc/CHANGES-staging/feat_audiosocket.txt

doc/CHANGES-staging/func_curl.txt

doc/CHANGES-staging/func_curl_headers.txt

doc/CHANGES-staging/func_jitterbuffer_video.txt