Update PR template

[jpnurmi]

I'd propose a drastically simpler PR template that should be helpful enough to the occasional new contributors but at the same time doesn't get in the way of the more frequent contributors.

<!-- REMINDER: If this PR introduces any visual changes, please run:
´´´
flutter test --update-goldens
´´´
and commit the changes **or** if not covered by tests, attach screenshots below:

|       | Before | After |
|-------|--------|-------|
| Light |        |       |
| Dark  |        |       |

-->

[Jupi007]

As I said in the other PR, I like this one, but if somebody who doesn't know well this repo want to contribute, it could be confusing for him.

So maybe do something a bit longer, but clearer:

<!--
REMINDER: if any design change, run `flutter test --update-goldens`,
**or** if not covered by golden tests attach screenshots below, please

|       | Before | After |
|-------|--------|-------|
| Light |        |       |
| Dark  |        |       |
-->

[Jupi007]

LGFM, you did it even better 👍

[jpnurmi]

I went 100x back and forth with the wording... Definitely one of the hardest PRs today 🤣