testing/synctest: explicitly state Run will be removed in Go 1.26

Updates #74012

Change-Id: I0407d9e87e9de6e02d3de0a462bc6a32c0faba4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/684875
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: qiu laidongfeng2 <2645477756@qq.com>

src/testing/synctest/run.go

@@ -10,7 +10,7 @@ import "internal/synctest"
 
 // Run is deprecated.
 //
-// Deprecated: Use Test instead.
+// Deprecated: Use Test instead. Run will be removed in Go 1.26.
 func Run(f func()) {
 	synctest.Run(f)
 }