Update module golang.org/x/sync to v0.15.0

This MR contains the following updates:

Package	Type	Update	Change
golang.org/x/sync	require	minor	v0.8.0 -> v0.15.0

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go.mod

 module git.autistici.org/ai3/tools/reports-collector
 
-go 1.21
+go 1.23.0
 
-toolchain go1.23.1
+toolchain go1.24.2
 
 require (
 	github.com/chrj/smtpd v0.3.1
 	github.com/jhillyerd/enmime v1.3.0
 	github.com/oschwald/geoip2-golang v1.11.0
 	github.com/prometheus/client_golang v1.20.3
-	golang.org/x/sync v0.8.0
+	golang.org/x/sync v0.13.0
 )
 
 require (