Rename extensionless PR templates across MDN #1970

What information was incorrect, unhelpful, or incomplete?

Some MDN repositories use an extensionless .github/PULL_REQUEST_TEMPLATE file, which can be mistaken for the documented PULL_REQUEST_TEMPLATE/ directory used for multiple templates.

What did you expect to see?

Rename these files across the MDN organization to .github/PULL_REQUEST_TEMPLATE.md, following GitHub's documented name for a single pull request template.

Do you have any supporting links, references, or citations?

GitHub Docs: Creating a pull request template

Do you have anything more you want to share?

No response