MAINTAINERS: update fsverity git repo, list, and patchwork

We're moving fsverity development to use its own git repo, mailing list,
and patchwork project, instead of reusing the fscrypt ones.  Update the
MAINTAINERS file accordingly.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230116232257.64377-1-ebiggers@kernel.org