Update issue templates

Pressing the "New issue" button leads to an endless loop:
- It shows a "Create new issue popup" with many options, the second and third to last are "Please open an issue in this repository!".
- Pressing one of these redirects to https://github.com/OAI/OpenAPI-Specification/issues/new/choose, which is a full-page version of the popup, and in turn only opens additional tabs with the same page.

I hope that adding a "Bug report" template will break that loop.