Switch to new Github Issue Template

[misaugstad]

It seems that we were using the legacy issue templates on Github. They are now transitioning to a new UI for Github issues, and support for the legacy issue templates that we were using has been dropped. So we'll need to switch to their new Issue template workflow:

https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/about-issue-and-pull-request-templates