Remove support for api versions as strings from KafkaAdmin
Remove support for api versions as strings from KafkaAdmin

This is a new class, so let's not support the old version strings and
saddle ourselves with tech debt right from the get-go.

DataDog/kafka-python,wikimedia/operations-debs-python-kafka,Aloomaio/kafka-python,Aloomaio/kafka-python,mumrah/kafka-python,dpkp/kafka-python,scrapinghub/kafka-python,ohmu/kafka-python,dpkp/kafka-python,Yelp/kafka-python,ohmu/kafka-python,mumrah/kafka-python,Yelp/kafka-python,wikimedia/operations-debs-python-kafka,scrapinghub/kafka-python