Serial.readBytesUntil() is either misleading or buggy

The method `Serial.readBytesUntil()` does not just read bytes until a delimiter but the part after the delimiter as well. This means either the name of the method is misleading or the method is buggy.

https://forum.arduino.cc/t/how-can-i-read-bytes-until-end-of-text-delimiter/898739/10

Start by locating the implementation and contract for Serial.readBytesUntil() in ArduinoCore-sam, since no file or test is named. Read the linked forum discussion to determine the intended delimiter behavior; done means the behavior or naming is unambiguous and covered by a regression test.

Written by the indexing model from the issue text.