BIP-341: explicitly allow softforks with future leaf versions

Currently the BIP-341 and BIP-342 leave the question of how to verify signature for non-`0xC0` leaf version scripts undefined. I haven't checked the Bitcoin Core code for that matter yet, but (1) I think we need to cover signature validation of non-`0xC0` leaf version scripts in this standard and (2) the only way of doing that is "always succeed" rule for the future leaf version values (otherwise we will need a hard fork to introduce them).

Specification