Simplify refinement template and add checklist

Why is this change being made?

This MR updates the refinement template to make it slicker. It moves to a checklist approach to help be more intentional about what questions need to be answered during refinement.

Additionally, it removes placeholders for relevant info that would better stay where it is easier to access them. E.g.:

• a list of MRs in the description instead of comments
• weight and status are reflected in the Issue sidebar and don't need to be in the refinement comment

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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