Please note that while it is possible to update the feed URL manually it is strongly discouraged to run such an outdated and end-of-life version of OpenVAS (now called GVM). Instead is is recommended to update to a still supported / recent version of GVM already using the new URLs (available since 6+ months).
OpenVAS-9 has already reached it’s end-of-life in September 2019 with GVM-10 and GVM-11 following by the end of this year (2020). This also means that the feed will be open for changes which are only available since GVM-20.08 and the feed is getting more and more incompatible with your outdated OpenVAS-9 and might even stop to work completely.