[process] Create PR template (#8099)

Replaces the (default) flutter template (which contains lots of
irrelevant detail) with a lighter-weight one inspired by what we have as
a generic template in the Dart SDK.

## Before this change:


<img width="1094" alt="image"
src="https://github.com/user-attachments/assets/747a66da-6617-414b-ae11-0506751a273f"
/>


## After this change:

<img width="1276" alt="image"
src="https://github.com/user-attachments/assets/7bfaffc7-e91b-4ae9-945c-808192a20b97"
/>


Inspiration from:
https://github.com/dart-lang/sdk/blob/main/.github/PULL_REQUEST_TEMPLATE.md

Fixes: #8062




## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md

Author: pq

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,14 @@
+- Thanks for your contribution! Please replace this text with a description of what this PR is changing or adding and why, list any relevant issues, and review the contribution guidelines below.
+
+---
+
+- [ ] I’ve reviewed the contributor guide and applied the relevant portions to this PR.
+
+<details>
+  <summary>Contribution guidelines:</summary><br>
+
+- See our [contributor guide]([https://github.com/dart-lang/sdk/blob/main/CONTRIBUTING.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview) for general expectations for PRs.
+- Larger or significant changes should be discussed in an issue before creating a PR.
+- Dart contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use `dart format`.
+- Java and Kotlin contributions should strive to follow Java and Kotlin best practices ([discussion](https://github.com/flutter/flutter-intellij/issues/8098)).
+</details>