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:

After this change:

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

Fixes: #8062

Pre-launch Checklist

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

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