switch to inbound=outbound for pull requests

According to GitHub [here](https://docs.github.com/en/github/site-policy/github-terms-of-service#6-contributions-under-repository-license)

> Whenever you add Content to a repository containing notice of a license, you license that Content under the same terms, and you agree that you have the right to license that Content under those terms. If you have a separate agreement to license that Content under different terms, such as a contributor license agreement, that agreement will supersede.
>
> Isn't this just how it works already? Yep. This is widely accepted as the norm in the open-source community; it's commonly referred to by the shorthand "inbound=outbound". We're just making it explicit.

Author: evancz