Use issue forms to augment existing templates

As [just announced](https://github.blog/changelog/2021-06-23-issues-forms-beta-for-public-repositories/), [issue forms](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository#creating-issue-forms) allow us to create smarter templates with form elements for our issues (and presumably PRs.

This could be useful for reducing friction and improving clarity of the existing templates. Some initial ideas for implementation:
- many fields are optional (e.g. OS on bug reports): this can be better communicated using
- Bevy version can be selected from a drop-down (each release, latest main, other)
- we could hook up some auto-labelling to form input; platform is the obvious suggestion. It's not clear that this is worthwhile though.

(cc @NathanSWard, @mockersf, @TheRawMeatball to continue discussion from Discord)

Review the repository's existing issue templates, then read the linked GitHub issue forms documentation. Decide which optional fields and Bevy version choices should become form elements, and whether platform input should drive labels. Done means the templates use the agreed forms and the labeling decision is resolved.

Written by the indexing model from the issue text.