fix: move pr template to lower directory

[maciejmajek]

What

This PR fixes the pull request template- it currently is not used automatically when a PR is created.

Why

Storing the pull request template inside the PULL_REQUEST_TEMPLATE folder forces the user to provide a query parameter to actually use the template.

docs.github.com

To create multiple pull request templates and use the template query parameter to specify a template to fill the pull request body, type .github/PULL_REQUEST_TEMPLATE/, then the name of your pull request template. For example, .github/PULL_REQUEST_TEMPLATE/pull_request_template.md. You can also store multiple pull request templates in a PULL_REQUEST_TEMPLATE subdirectory within the root or docs/ directories. For more information, see "Using query parameters to create a pull request."