Update CHANGES and UPGRADE.txt for 20.0.0

doc/CHANGES-staging/app_read.txt

doc/CHANGES-staging/app_sendtext.txt

doc/CHANGES-staging/app_voicemail.txt

doc/CHANGES-staging/app_voicemail_danish_syntax.txt

doc/CHANGES-staging/app_voicemail_nodelete.txt

doc/CHANGES-staging/ari_add_pvt_id_to_channel_resource.txt

doc/CHANGES-staging/ast_coredumper.txt

doc/CHANGES-staging/bundled-pjproject-build.txt

doc/CHANGES-staging/cdr_disable.txt

doc/CHANGES-staging/chan_dahdi_cadences.txt

doc/CHANGES-staging/chan_dahdi_polarity.txt

doc/CHANGES-staging/chan_iax2_ani2.txt

doc/CHANGES-staging/chan_iax2_dial.txt

doc/CHANGES-staging/chan_pjsip_180_sdp.txt

doc/CHANGES-staging/chan_pjsip_flash.txt

doc/CHANGES-staging/chan_sip_pickup_AST_STATE_DOWN.txt

doc/CHANGES-staging/chan_sip_session-timer_on_update.txt

doc/CHANGES-staging/channel_internal_api.txt

doc/CHANGES-staging/cli_eval_function.txt

doc/CHANGES-staging/cli_refresh.txt