[core] reduce trace on Upgrade backend connection
[core] reduce trace on Upgrade backend connection

reduce trace on Upgrade'd backend connection when ECONNRESET received,
which, for example, apparently might occur if a backend calls close()
on socket without first calling shutdown(fd, SHUT_WR) -- seen on Linux
kernel 5.16.15 where lighttpd received ECONNRESET when trying to read()
(instead of receiving EOF).

lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4