Shorten PR template

[smelc]

This PR changes the PR template so that it looks like below. The rationale is that right now most developers don't use the PR list of checkboxes, so I have:

• Removed all checkboxes that are enforced by CI anyway
• Kept only checkboxes that are good practices reminders
• I have added a How to trust this PR section In my experience, this section can entice developers to help the reviewers, which is important, because reviews is often a bottleneck we're all blocked on regularly 🙂

New PR template

Changelog

- description: |
    Change PR template
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - improvement    # QoL changes e.g. refactoring
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Additional context for the PR goes here. If the PR fixes a particular issue please provide a link to the issue.

How to trust this PR

Highlight important bits of the PR that will make the review faster. If there are commands the reviewer can run to observe the new behavior, describe them.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff