* sigstore: 0.6.3

Signed-off-by: William Woodruff <william@trailofbits.com>

* README: update GitHub Action version

Signed-off-by: William Woodruff <william@trailofbits.com>

* README: Correct `gh-action-sigstore-python` version

Signed-off-by: Alex Cameron <alex.cameron@trailofbits.com>

Co-authored-by: Alex Cameron <asc@tetsuo.sh>