message: Update PR template to use headings
diff: <!-- Please remember to update CHANGELOG.md in the root of the project if you have not done so. -->
  -**Summary**
  +## Summary
  <!-- Explain the **motivation** for making this change. What existing problem does the pull request solve? -->
  -**Test plan**
  +## Test plan
  <!-- Demonstrate the code is solid. Example: The exact commands you ran and their output, screenshots / videos if the pull request changes UI. -->
  