Change clients to automatically use IPv6 if they can bootstrap over it

Tor currently defaults to avoiding IPv6 directories and ORPorts. When legacy/trac#8374 (moved) and legacy/trac#4483 (moved) are implemented, we should change this default, especially for those clients that can bootstrap successfully over IPv6.

I suggest we add an "auto" option, and make it the default. It would mean that the client uses whichever IP version it bootstrapped over.

ClientUseIPv6 0
ClientPreferIPv6ORPort 0

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