Make BehaviorVersion as future-proof as the documentation and RFC specify (#3513)

Fixes aws-sdk-rust#1111

This is *technically* a breaking change but since this was released
recently and it's documented that this API shouldn't be used, this can
probably be considered a bug, which is better fixed now than later.

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Co-authored-by: ysaito1001 <awsaito@amazon.com>