docs: Update PR template

[ianlewis]

Description:

Updates the PR template to tweak some of text and add an item to the checklist to update the CHANGELOG.

Checklist:

• Review the CONTRIBUTING.md documentation.
• A reference to a related issue in the repository.
• A description of the changes proposed in the pull request.
• Please make sure to sign the Google CLA.
• Please make sure your commits include a Developer Certificate of Origin signoff.
• Please add unit tests

[codecov]

Codecov Report

Merging #544 (09f3fb9) into main (5c4ac57) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #544   +/-   ##
=======================================
  Coverage   80.27%   80.27%           
=======================================
  Files          19       19           
  Lines        1110     1110           
  Branches       14       14           
=======================================
  Hits          891      891           
  Misses        157      157           
  Partials       62       62