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>

tstest/natlab/natlab.go

@@ -18,6 +18,7 @@ import (
 	"net"
 	"net/netip"
 	"os"
+	"slices"
 	"sort"
 	"strconv"
 	"sync"
@@ -247,12 +248,7 @@ func (f *Interface) String() string {
 
 // Contains reports whether f contains ip as an IP.
 func (f *Interface) Contains(ip netip.Addr) bool {
-	for _, v := range f.ips {
-		if ip == v {
-			return true
-		}
-	}
-	return false
+	return slices.Contains(f.ips, ip)
 }
 
 type routeEntry struct {
@@ -348,10 +344,8 @@ func (m *Machine) isLocalIP(ip netip.Addr) bool {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	for _, intf := range m.interfaces {
-		for _, iip := range intf.ips {
-			if ip == iip {
-				return true
-			}
+		if slices.Contains(intf.ips, ip) {
+			return true
 		}
 	}
 	return false
@@ -565,7 +559,7 @@ func (m *Machine) interfaceForIP(ip netip.Addr) (*Interface, error) {
 func (m *Machine) pickEphemPort() (port uint16, err error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	for tries := 0; tries < 500; tries++ {
+	for range 500 {
 		port := uint16(rand.IntN(32<<10) + 32<<10)
 		if !m.portInUseLocked(port) {
 			return port, nil

tstest/natlab/vnet/vnet.go

@@ -1917,7 +1917,7 @@ func (n *network) doPortMap(src netip.Addr, dstLANPort, wantExtPort uint16, sec
 		}
 	}
 
-	for try := 0; try < 20_000; try++ {
+	for range 20_000 {
 		if wanAP.Port() > 0 && !n.natTable.IsPublicPortUsed(wanAP) {
 			mak.Set(&n.portMap, wanAP, portMapping{
 				dst:    dst,