Fix issue/PR templates (#9117)

* Unwrap text in templates

GitHub markdown does not do auto-unwrapping.

* Switch to one-template PR

This Stack Overflow answer [1] details how to have multiple PR templates
on GitHub. The UX is not that immediate (user has to click on preview, and
then another link).
A single page with two templates (where you fill in the one you
pick and erase the other) is more friendly.

[1] https://stackoverflow.com/a/75030350

* Whitespace

* Made PR instruction impersonal

For consistency with other bullet points.