Change log about missing descriptor's observed bandwidth

As @sebastian reported by irc, sbws logs Jan 02 05:41:20 v3bwfile[377088]: <WARNING> (MainThread) Descriptor observed bandwidth is None.

That line was added in #33871 (closed) to report the case and do not scale relay's missing that bandwidth value.

It's possible that this case happen cause sbws is configured to gett "early", "useless" descriptors or it doesn't check the running (and others) flag or relay in dormant mode (#33166 (moved)).

I'd create a different issue to try to figure out why this happen in a later version and for now lower the level to info so that there aren't too many email notifications when configuring sbws as explained in #40110 (closed).