build(deps): update module golang.org/x/sync to v0.21.0

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
golang.org/x/sync	v0.20.0 → v0.21.0

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• ✅ Entered queue — 2026-06-25 09:10 UTC · Rule: renovate · triggered by rule Automatic merge on approval
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• ✅ Merged — 2026-06-25 09:20 UTC · at dad5f2549eaebfbb0e2996962e6a754c2bbf1d49

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