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>

cmd/containerboot/forwarding.go

@@ -51,7 +51,7 @@ func ensureIPForwarding(root, clusterProxyTargetIP, tailnetTargetIP, tailnetTarg
 		v4Forwarding = true
 	}
 	if routes != nil && *routes != "" {
-		for _, route := range strings.Split(*routes, ",") {
+		for route := range strings.SplitSeq(*routes, ",") {
 			cidr, err := netip.ParsePrefix(route)
 			if err != nil {
 				return fmt.Errorf("invalid subnet route: %v", err)