Each subfield is now a separate column as indicated by the answers prefix, and the text field is a list now. Instead
of tokenizing each sentence separately, convert the list to a string so you can jointly tokenize them.
Here is a first preprocessing function to join the list of strings for each example and tokenize the result:

def preprocess_function(examples):
     return tokenizer([" ".join(x) for x in examples["answers.text"]])