`orchard 0.10.1` depends on `subtle 2.6`

[str4d]

We currently require subtle 2.3: https://github.com/zcash/orchard/blob/9afad02db07d903d3f21b493d51c31b314841746/Cargo.toml#L43 However in 0.10.1 we started using CtOption::into_option (in the pczt module), which was added in subtle 2.6. We should either remove our usages of it, or raise our subtle requirement to 2.6 (which would make it incompatible in the same codebase as bip0039 0.10 as that depends on password-hash 0.3 which has subtle < 2.5 for annoying old MSRV reasons).