Revert back default PR template

[Kyle-Ye]

Bug/issue #, if applicable:

Summary

After #300 was merged, we lose the ability to use a default PR template when creating PR directly in Github website.

See more discussion here. https://github.community/t/is-there-a-pull-request-template-selector-similar-to-issues

Dependencies

None

Testing

None

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[franklinsch]

Given this, I'd prefer us combining the two templates into one so that they're more visible. See #300 (comment)

[Kyle-Ye]

templates

What if we have more than 2 templates? I think we should follow the Github spec instead of putting them in one file. See all the reasons I list on #300. Given that we lose discoverability of the cherry-pick template, we can add some comment in the header in default template.

If you're opening a PR to cherry-pick a change for a release branch, use this template instead:
https://github.com/apple/swift-docc/blob/main/.github/PULL_REQUEST_TEMPLATE/CHERRY_PICK.md

Curretnly, Github website does not have a UX for it.
You can manually set the template on qurey parmas eg. https://github.com/apple/swift->docc/compare/release/5.7...apple:main?template=CHERRY_PICK.md
or use GitHub CLI - gh to choose all the PR templates.

[franklinsch]

Thanks for working through this @Kyle-Ye, I really appreciate it 🙏

[Kyle-Ye]

@swift-ci please test

[Kyle-Ye]

The default PR template is back now.

By the way, can we make some progress on #268?