chore(deps): bump golang.org/x/sync from 0.20.0 to 0.21.0

Bumps [golang.org/x/sync](https://github.com/golang/sync) from 0.20.0 to 0.21.0.
- [Commits](golang/sync@v0.20.0...v0.21.0)

---
updated-dependencies:
- dependency-name: golang.org/x/sync
  dependency-version: 0.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

go.mod

@@ -4,5 +4,5 @@ go 1.25.0
 
 require (
 	github.com/toon-format/toon-go v0.0.0-20251202084852-7ca0e27c4e8c
-	golang.org/x/sync v0.20.0
+	golang.org/x/sync v0.21.0
 )

go.sum

@@ -1,4 +1,4 @@
 github.com/toon-format/toon-go v0.0.0-20251202084852-7ca0e27c4e8c h1:D8lDFovBMZywze1eh9iwMLcYor5f11mHBocLhO7cBe8=
 github.com/toon-format/toon-go v0.0.0-20251202084852-7ca0e27c4e8c/go.mod h1:j/BOnpF2ihnz4lELs99h9mwGJBx/zdleOUCnLLRPCsc=
-golang.org/x/sync v0.20.0 h1:e0PTpb7pjO8GAtTs2dQ6jYa5BWYlMuX047Dco/pItO4=
-golang.org/x/sync v0.20.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=
+golang.org/x/sync v0.21.0 h1:HLII4xRRTtCRkxYp4HNFF0Js/Og6q2i++KXbg0gHCwM=
+golang.org/x/sync v0.21.0/go.mod h1:9xrNwdLfx4jkKbNva9FpL6vEN7evnE43NNNJQ2LF3+0=