updating openpgp-card crate.

I'm currently looking at upgrading the "secrecy" crate in Debian from 0.8 to 0.10, one of it's reverse-dependencies is the openpgp-card crate, and it seems to make sense to update this to 0.6 at the same time.

I ran the cargo tests for sequoia-keystore-openpgp-card with the dependency bumped and they passed, but I don't have an openpgp card myself, and the maintainers of the openpgp-card crate did not provide a useful changelog.

Any opinions on the upgrade would be appreciated.