#6306 Update PR template

[martgil]

This PR only updates the PR template, from which it adds extra steps for checking PR reviews.

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Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)
• Difficult to test (explain why)
• Not worth testing
• Tests will be added later (issue #...)
• Tests added or updated

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[martgil]

@sosnovsky - This one is ready for a review. Thanks!