Simplify PR template (free-text sections instead of checkboxes)

[xen2]

Simplifies the PR template so pull requests no longer show an incomplete task counter ("0 of 8 tasks") in the PR list.
Every - [ ] in a description counts toward that badge, and since almost nobody ticks all the boxes, every PR looked unfinished forever.

Changes:

• Replaced the "Types of changes" and "Checklist" checkbox lists with free-text sections.
• Removed "All new and existing tests passed", tests now run on every PR in CI.
• Kept the important signals as prompts instead: issue link (with a Fixes #1234 hint), how the change was tested (editor changes should still be tried in the editor, CI does not cover it fully), breaking changes and documentation impact.

How did you test this?

No code change, github template only.

[Kryptos-FR]

tests now run on every PR in CI

Only if the workflows are approved, which is not by default for non-members. So if we remove that, there should be a check that is enforced and blocks the PR until the workflows run.