pull request template: cleanup (#117537)

Remove elements of the PR template that have a low signal/noise ratio,
and add one that I think would have a good signal/noise ratio.

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Remove:
Determined the impact on package closure size (by running `nix path-info
-S` before and after)
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Rationale:
This is rarely done in practice, and apart from for specific packages
this is usually not a good indicator of anything useful

It might make sense to re-introduce it with two holes to fill, but then
we would have to make a serious decision to never land without these two
numbers filled in or with too big a regression, because in practice this
box has been a no-op in many cases.

Maybe just integrating this check in nixpkgs-review would bring the most
benefit here?
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Remove:
Ensured that relevant documentation is up to date
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Rationale:
This is fuzzy, “relevant documentation” is way too often hard to find
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Add:
Added a release notes entry if the change is major or breaking
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Rationale:
This is way too often forgotten, and is also a self-contained easy task
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