Reject non-UTF-8 documents everwhere, and close prop285.

Currently, dirauths reject non-utf-8 documents. We should make them rejected everywhere.

Per torspec!342 (merged) and discussion at torspec#296 (closed), it is okay to make our parsers a bit stricter. In this case, doing so might actually limit our implementation fingerprintability, since it will make c tor a bit more like arti.

Once we've made this change, we should update tor-spec/spec/dir-spec/netdoc.md accordingly.