Adopt new GitHub Issues Forms syntax

[Kyle-Ye]

Bug/issue #, if applicable: #232

Summary

Migrate existing templates over to the new format to make it easier for contributors to file bug reports and feature requests.

Test

1. The yaml templet feature is currently in beta and only available on a public repo.
2. "Issue" feature can't be tested on a forked repo. (We can't simply test it like what we did when we add swift-docc support for deploying docs to GitHub page)

Considering above, I have to create an empty & public repo to test this PR.
https://github.com/Kyle-Ye/Issue/issues/new/choose

[Kyle-Ye]

@ethan-kusters Could you help review this?

[franklinsch]

@ethan-kusters any thoughts here? I'm not very familiar with Forms

[Kyle-Ye]

Ping @ethan-kusters. Could you help spare some time to review this?

[ethan-kusters]

Thank you so much for investigating and opening this Kyle! Having the preview over on https://github.com/Kyle-Ye/Issue/issues was very helpful.

I just left a few thoughts on the overall formatting but I think this is already a huge improvement over what we have.

[Kyle-Ye]

Resolve the comments you provide above @ethan-kusters.

And you can check the updated preview over here https://github.com/Kyle-Ye/Issue/issues/new/choose

[ethan-kusters]

This is looking great- just had a couple of last thoughts. Thank you!

[Kyle-Ye]

Review notes for @ethan-kusters

1. The render key is not supported on type: input
2. The placeholder value: '5.6.0' for example, or a commit hash must be quoted for some reason. Otherwise, we'll get a yaml parse error.

[Kyle-Ye]

@swift-ci please test

[ethan-kusters]

Looks great. Thank you so much for your work on this @Kyle-Ye!

[franklinsch]

Just came across this when creating a new issue, so cool! Thanks @Kyle-Ye!