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>

net/art/stride_table_test.go

@@ -19,7 +19,7 @@ import (
 func TestInversePrefix(t *testing.T) {
 	t.Parallel()
 	for i := range 256 {
-		for len := 0; len < 9; len++ {
+		for len := range 9 {
 			addr := i & (0xFF << (8 - len))
 			idx := prefixIndex(uint8(addr), len)
 			addr2, len2 := inversePrefixIndex(idx)