[3.6] Correct TooManyLogicalSessions error code to be consistent with 4.0.1

A backport to 3.6 for SERVER-33697 renumbered the named error code TooManyLogicalSessions to 260, which is not consistent with master or v4.0. We should correct this so it is consistent with v4.0 and set it to 261: https://github.com/mongodb/mongo/blob/869931050527d8d0b6955490bb972021151c00b1/src/mongo/base/error_codes.err#L261-L263