all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

syncs/shardvalue_test.go

@@ -66,10 +66,10 @@ func TestShardValue(t *testing.T) {
 		iterations := 10000
 		var wg sync.WaitGroup
 		wg.Add(goroutines)
-		for i := 0; i < goroutines; i++ {
+		for range goroutines {
 			go func() {
 				defer wg.Done()
-				for i := 0; i < iterations; i++ {
+				for range iterations {
 					sv.One(func(v *intVal) {
 						v.Add(1)
 					})