[dart] Bump min dio version to prevent unused import

[kuhnroyal]

Just a minor bump to prevent some import warnings.

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CC @jaumard (2018/09) @josh-burton (2019/12) @amondnet (2019/12) @sbu-WBT (2020/12) @kuhnroyal (2020/12) @agilob (2020/12) @ahmednfwela (2021/08)

[agilob]

👍