[Pull Request Template] Moved 'User Impact' query to a comment instead of checkbox

[rcaril]

Change summary

This PR updates the Github Pull Request template from:

To:

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

User Impact

• What is the user impact of this change?
  As you can see, having a checkbox here doesn't make much sense. Moving this clarification to a comment instead is more logical.