syncs: delete WaitGroup and use sync.WaitGroup.Go in Go 1.25

Our own WaitGroup wrapper type was a prototype implementation
for the Go method on the standard sync.WaitGroup type.
Now that there is first-class support for Go,
we should migrate over to using it and delete syncs.WaitGroup.

Updates #cleanup
Updates tailscale/tailscale#16330

Change-Id: Ib52b10f9847341ce29b4ca0da927dc9321691235
Signed-off-by: Joe Tsai <joetsai@digital-static.net>

syncs/syncs_test.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"io"
 	"os"
+	"sync"
 	"testing"
 	"time"
 
@@ -98,7 +99,7 @@ func TestMutexValue(t *testing.T) {
 		t.Errorf("Load = %v, want %v", v.Load(), now)
 	}
 
-	var group WaitGroup
+	var group sync.WaitGroup
 	var v2 MutexValue[int]
 	var sum int
 	for i := range 10 {
@@ -237,7 +238,7 @@ func TestMap(t *testing.T) {
 
 	t.Run("LoadOrStore", func(t *testing.T) {
 		var m Map[string, string]
-		var wg WaitGroup
+		var wg sync.WaitGroup
 		var ok1, ok2 bool
 		wg.Go(func() { _, ok1 = m.LoadOrStore("", "") })
 		wg.Go(func() { _, ok2 = m.LoadOrStore("", "") })