Migrate templates for issues and PRs to new style

[magnusbaeck]

Description

Nowadays a repository can have multiple templates for issues and PRs, see https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/about-issue-and-pull-request-templates. While we currently don't have more than one template for either type of entity this could be useful in the future. In fact, it already is in the community repository; see eiffel-community/community#101.

Motivation

Using the new style of templates in this repository would enable a few more features apart from the ability to have multiple templates, and for other eiffel-community repositories that add their own templates it might be easier to understand how they interact with the ones in this repository if they use the same style.

Exemplification

It's totally conceivable that we'll want to have separate issue templates for bug reports and feature requests, and that's not possible with the legacy template style.

Benefits

More features that make our repositories friendlier and more convenient to work with.

Possible Drawbacks

None.