Update module golang.org/x/sync to v0.3.0

This MR contains the following updates:

Package	Type	Update	Change
golang.org/x/sync	require	minor	v0.1.0 -> v0.3.0

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go.mod

@@ -7,5 +7,5 @@ require (
 	github.com/jhillyerd/enmime v0.11.0
 	github.com/oschwald/geoip2-golang v1.8.0
 	github.com/prometheus/client_golang v1.11.0
-	golang.org/x/sync v0.1.0
+	golang.org/x/sync v0.3.0
 )