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/socks5/socks5.go

@@ -21,6 +21,7 @@ import (
 	"io"
 	"log"
 	"net"
+	"slices"
 	"strconv"
 	"time"
 
@@ -488,10 +489,8 @@ func parseClientGreeting(r io.Reader, authMethod byte) error {
 	if err != nil {
 		return fmt.Errorf("could not read methods")
 	}
-	for _, m := range methods {
-		if m == authMethod {
-			return nil
-		}
+	if slices.Contains(methods, authMethod) {
+		return nil
 	}
 	return fmt.Errorf("no acceptable auth methods")
 }

net/socks5/socks5_test.go

@@ -180,11 +180,11 @@ func TestUDP(t *testing.T) {
 
 	const echoServerNumber = 3
 	echoServerListener := make([]net.PacketConn, echoServerNumber)
-	for i := 0; i < echoServerNumber; i++ {
+	for i := range echoServerNumber {
 		echoServerListener[i] = newUDPEchoServer()
 	}
 	defer func() {
-		for i := 0; i < echoServerNumber; i++ {
+		for i := range echoServerNumber {
 			_ = echoServerListener[i].Close()
 		}
 	}()
@@ -277,10 +277,10 @@ func TestUDP(t *testing.T) {
 	}
 	defer socks5UDPConn.Close()
 
-	for i := 0; i < echoServerNumber; i++ {
+	for i := range echoServerNumber {
 		port := echoServerListener[i].LocalAddr().(*net.UDPAddr).Port
 		addr := socksAddr{addrType: ipv4, addr: "127.0.0.1", port: uint16(port)}
-		requestBody := []byte(fmt.Sprintf("Test %d", i))
+		requestBody := fmt.Appendf(nil, "Test %d", i)
 		responseBody := sendUDPAndWaitResponse(socks5UDPConn, addr, requestBody)
 		if !bytes.Equal(requestBody, responseBody) {
 			t.Fatalf("got: %q want: %q", responseBody, requestBody)