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/speedtest/speedtest.go

@@ -72,8 +72,7 @@ var speedtestArgs struct {
 func runSpeedtest(ctx context.Context, args []string) error {
 
 	if _, _, err := net.SplitHostPort(speedtestArgs.host); err != nil {
-		var addrErr *net.AddrError
-		if errors.As(err, &addrErr) && addrErr.Err == "missing port in address" {
+		if addrErr, ok := errors.AsType[*net.AddrError](err); ok && addrErr.Err == "missing port in address" {
 			// if no port is provided, append the default port
 			speedtestArgs.host = net.JoinHostPort(speedtestArgs.host, strconv.Itoa(speedtest.DefaultPort))
 		}