fix: downgrade flutter version to v3.16.8

[monkeyWie]

After upgrading the Flutter version to v3.24, several serious bugs have appeared, so a downgrade to v3.16, related issues:

• https://github.com/GopeedLab/gopeed/issues/818
• https://github.com/GopeedLab/gopeed/issues/814

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 58.69%. Comparing base (cd409fc) to head (f7fa96b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #820 +/- ## ========================================== - Coverage 58.89% 58.69% -0.21% ========================================== Files 35 35 Lines 4464 4464 ========================================== - Hits 2629 2620 -9 - Misses 1559 1571 +12 + Partials 276 273 -3 ```

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