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>

ipn/serve.go

@@ -673,7 +673,7 @@ func CheckFunnelPort(wantedPort uint16, node *ipnstate.PeerStatus) error {
 		return deny("")
 	}
 	wantedPortString := strconv.Itoa(int(wantedPort))
-	for _, ps := range strings.Split(portsStr, ",") {
+	for ps := range strings.SplitSeq(portsStr, ",") {
 		if ps == "" {
 			continue
 		}