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/neterror/neterror_linux.go

@@ -12,8 +12,7 @@ import (
 
 func init() {
 	shouldDisableUDPGSO = func(err error) bool {
-		var serr *os.SyscallError
-		if errors.As(err, &serr) {
+		if serr, ok := errors.AsType[*os.SyscallError](err); ok {
 			// EIO is returned by udp_send_skb() if the device driver does not
 			// have tx checksumming enabled, which is a hard requirement of
 			// UDP_SEGMENT. See: