Improve PR template (#220)

## Fixes / Closes (optional)

None.

## Description

Some improvements in the PR template.

## Summary (check all that apply)

- [ ] Modified / added code
- [ ] Modified / added tests
- [ ] Modified / added examples
- [x] Modified / added others (pubspec.yaml, workflows, etc...)
- [ ] Updated README
- [ ] Contains breaking changes
  - [ ] Created / updated migration guide
- [ ] Incremented version number
  - [ ] Updated CHANGELOG

.github/pull_request_template.md

@@ -1,15 +1,13 @@
-## Fixes / Closes (optional)
-<!-- List any issues or pull requests that this PR fixes or closes. Use the format: "Fixes #123" or "Closes #456". -->
+## Related issues (optional)
 
 None.
 
 ## Description
-<!-- Provide a clear and concise description of what this PR does. Explain the problem it solves and the approach you've taken. -->
 
 Blah blah blah.
 
 ## Summary (check all that apply)
-<!-- Mark the boxes that apply to this PR. Add details if necessary. -->
+
 - [ ] Modified / added code
 - [ ] Modified / added tests
 - [ ] Modified / added examples