Update PR template

[coolreader18]

Description of Changes

This has been annoying me for months haha. This turns the instructions for each section into <!-- comments -->, so that they don't show up in the rendered description if they're not removed, and it makes it much more obvious to see when a section isn't filled out because there's nothing there. Also removed the default X from the testing checkbox, so that the PR author has to actually check it to show they've done it. Also, maybe the "API and ABI breaking changes" should also just be a comment?

Testing

• [x] Tested how it renders. The below is the new template, rendered but unedited:

Description of Changes

API and ABI breaking changes

Expected complexity level and risk

Testing

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