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	v0.17.0 → v0.21.0

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[renovate]

⚠️ Artifact update problem

Renovate failed to update an artifact related to this branch. You probably do not want to merge this PR as-is.

♻ Renovate will retry this branch, including artifacts, only when one of the following happens:

• any of the package files in this branch needs updating, or
• the branch becomes conflicted, or
• you click the rebase/retry checkbox if found above, or
• you rename this PR's title to start with "rebase!" to trigger it manually

The artifact failure details are included below:

File name: .dagger/postgresdb/go.sum

Command failed: go get -t ./...
go: module golang.org/x/sync@v0.21.0 requires go >= 1.25.0; switching to go1.25.11
go: downloading go1.25.11 (linux/amd64)
go: dagger/postgresdb imports
	dagger/postgresdb/internal/dagger: package dagger/postgresdb/internal/dagger is not in std (/runner/cache/others/go/pkg/mod/golang.org/toolchain@v0.0.1-go1.25.11.linux-amd64/src/dagger/postgresdb/internal/dagger)