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/tstun/wrap.go

@@ -1406,11 +1406,11 @@ func (t *Wrapper) InjectInboundPacketBuffer(pkt *netstack_PacketBuffer, buffs []
 			return err
 		}
 	}
-	for i := 0; i < n; i++ {
+	for i := range n {
 		buffs[i] = buffs[i][:PacketStartOffset+sizes[i]]
 	}
 	defer func() {
-		for i := 0; i < n; i++ {
+		for i := range n {
 			buffs[i] = buffs[i][:cap(buffs[i])]
 		}
 	}()

net/tstun/wrap_test.go

@@ -95,7 +95,7 @@ func tcp4syn(src, dst string, sport, dport uint16) []byte {
 
 func nets(nets ...string) (ret []netip.Prefix) {
 	for _, s := range nets {
-		if i := strings.IndexByte(s, '/'); i == -1 {
+		if found := strings.Contains(s, "/"); !found {
 			ip, err := netip.ParseAddr(s)
 			if err != nil {
 				panic(err)
@@ -122,13 +122,13 @@ func ports(s string) filter.PortRange {
 	}
 
 	var fs, ls string
-	i := strings.IndexByte(s, '-')
-	if i == -1 {
+	before, after, ok := strings.Cut(s, "-")
+	if !ok {
 		fs = s
 		ls = fs
 	} else {
-		fs = s[:i]
-		ls = s[i+1:]
+		fs = before
+		ls = after
 	}
 	first, err := strconv.ParseInt(fs, 10, 16)
 	if err != nil {