use `span.context.http.response.status_code` if talking to APM Server >=7.7

[trentm]

span.context.http.status_code was deprecated in favour of span.context.http.response.status_code in APM Server 7.7: https://github.com/elastic/apm-server/blob/master/changelogs/7.7.asciidoc#intake-api-changes

Now that we have APM Server version sniffing (as of #2546), we should be able to know when to make this change.