circuit_can_use_tap() should only allow TAP for v2 onion services
circuit_can_use_tap() checks the circuit purpose to make sure that it's an onion service circuit. But it should also check that the circuit is for a v2 onion service before allowing TAP.
There should be a field in the circuit or extend_info that we can use for this.
This is security-low, because it's a defence in depth mechanism that doesn't provide as much defence as we thought.
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