Improve the PR template

[halldorfannar]

There are a few goals with this change, and I list them below. Please suggest tweaks or better wording. English is my third language 😄

• Make the PR template simpler
  • remove the heading because every PR has a Title that is outside this template
  • instead of the "open to interpretation" (...) we now have a comment that tells contributors what we expect
• Make the template reflect our new reality
  • We now have release notes instead of a changelog. We probably need more docs about how contributors are expected to update the release notes. Maybe those guides exist and I just couldn't find them.
• Drive contributors to take action on all items
  • before they would only check if a test was added/update, etc but this makes the GitHub interface look odd because a PR will show as having incomplete tasks.
  • similarly, with the changed wording for AI disclosure we can tell if people took action there or not. If someone doesn't disclose we will assume AI was at least partially involved.

• Tests added/updated (or your review concluded: not needed)
• Documentation updated (or your review concluded: not needed)
• Release notes added (or your review concluded: not needed)
• ./check.sh passed

AI assistance disclosure

• AI was not used to create this change.

[halldorfannar]

Created this from the wrong base branch. Will close and create another PR.