Finding latest release assumes latest tag is on master

I believe the current way of finding the latest tag in `update-submodules.sh` will find the latest tag on any branch, not just on `master`. For libraries, we never tag other than on `master`, but some day, we might. (We tag on branches in https://github.com/adafruit/circuitpython, for instance.)

For many suggestions about finding the latest tag on a branch, see https://stackoverflow.com/questions/1404796/how-to-get-the-latest-tag-name-in-current-branch-in-git