contributing: adjust MR template to be less verbose

Magnusrequested to merge
wip/mangoiv/revise-merge-template into master

Changes contained in this patch

This MR adds a changed template that only shows the bits that are relevant in the final markdown.

This MR was created using the changed template. (Reviewers press edit to see how it looks in the gl UI)

MR Checklist

  • This MR solves the problem described in the following issue: #27165 (closed)
  • I added a changelog entry in changelog.d/ for user-facing changes (see changelog guide). If this MR does not need a changelog entry, I applied the no-changelog label.
  • This MR does not make any significant changes to base or I have risen a CLC proposal.
  • I made sure this MR did not break any existing programs (e.g. rejecting previously accepted programs) and otherwise applied the user-facing label.
  • I made sure that my commits are either individually buildable or squashed.
  • I ensured that my commit messages describe what they do, referring to tickets using #NNNNN syntax.
  • I added source comments describing my change. For larger changes I added a notes and cross-referenced it from the relevant places.
  • I added testcases to the testsuite as applicable.
  • I updated the users guide as applicable.

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