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>

util/slicesx/slicesx_test.go

@@ -53,7 +53,7 @@ func TestShuffle(t *testing.T) {
 	}
 
 	var wasShuffled bool
-	for try := 0; try < 10; try++ {
+	for range 10 {
 		shuffled := slices.Clone(sl)
 		Shuffle(shuffled)
 		if !reflect.DeepEqual(shuffled, sl) {