Add script to update the api version

[rattrayalex-stripe]

r? @ob-stripe
cc @stripe/api-libraries @remi-stripe

This automates the work done in #826 with a handy little regex script (and a new API_VERSION file, which is codegen'd).

It also:

1. Removes Addresses.d.ts, which is obsolete – the definition for type Address lives in tyeps/shared.d.ts. I misadvised remi when I told him we need to keep it, I think.
2. Updates the openapi spec, because it's hard not to.
3. Add (and revert) a demo commit to show what would happen if you ran this for api version 2020-12-12: 059273c

[ob-stripe]

Nice! LGTM, but I think this may conflict with #842, be careful when merging.

[rattrayalex-stripe]

Awesome thanks – I'll merge & release that one first.

[rattrayalex-stripe]

I rebased and dropped the openapi update, since remi's PR should have been ahead of that if anything.