Fix COPY FROM when database encoding is SQL_ASCII.
In the codepath when no encoding conversion is required, the check for
incomplete character at the end of input incorrectly used server
encoding's max character length, instead of the client's. Usually the
server and client encodings are the same when we're not performing
encoding conversion, but SQL_ASCII is an exception.
In the passing, also fix some outdated comments that still talked about
the old COPY protocol. It was removed in v14.
Per bug #17501 from Vitaly Voronov. Backpatch to v14 where this was
introduced.
Discussion: https://www.postgresql.org/message-id/17501-
128b1dd039362ae6@postgresql.org